Add documentation adjust code (RH dists and internal cleanup)

[thiagoftsm]

Summary

This PR is bringing some "small" (RH modified kernel 5.14 and 4.18, so I had to push an additional commit for it.) updates in the code and a documentation to help new eBPF developers (Welcome @stelfrag !).

Test Plan

1. Get binaries according your LIBC from this link and extract them inside a directory.
2. Extract them :

   for i in `ls *.zip`; do unzip $i; rm .gitkeep ; rm $i; done
   for i in `ls *.xz`; do tar -xf $i; rm $i* ; done

3. Compile branch an run the following tests:

   # make; make tester
   # for i in `seq 0 2`; do ./kernel/legacy_test --netdata-path ../directory --content --iteration --pid $i --log-path file_pid$i.txt; done

4. Every test should finally with Success, unless you do not have a specific target (function) available.

   Additional information

This PR was tested on:

Linux Distribution	kernel version	real parent	parent	all
Slackware current	6.1.11	slackware_6_11_pid0.txt	slackware_6_11_pid1.txt	slackware_6_11_pid2.txt
Arch Linux	6.1.9-arch1-1	arch_6_9_pid0.txt	arch_6_9_pid1.txt	arch_6_9_pid2.txt
Ubuntu 22.04	5.15.0-56-generic	ubuntu_5_15_pid0.txt	ubuntu_5_15_pid1.txt	ubuntu_5_15_pid2.txt
Alma 9	5.14.0-162.12.1.el9_1.x86_64	alma_5_14_pid0.txt	alma_5_14_pid1.txt	alma_5_14_pid2.txt
Debian 11	5.10.0-21-amd64	debian_5_10_pid0.txt	debian_5_10_pid1.txt	debian_5_10_pid2.txt
Slackware Current	5.4.210	slackware_5_4_pid0.txt	slackware_5_4_pid1.txt	slackware_5_4_pid2.txt
Oracle 8.6*	5.4.17-2136.313.6.el8uek.x86_64	oracle_5_4_pid0.txt	oracle_5_4_pid1.txt	oracle_5_4_pid2.txt
Alma 8.6	4.18.0-425.3.1.el8.x86_64	alma_4_18_pid0.txt	alma_4_18_pid1.txt	alma_4_18_pid2.txt
Ubuntu 18.04	4.15.0-180-generic	ubuntu_4_15_pid0.txt	ubuntu_4_15_pid1.txt	ubuntu_4_15_pid2.txt
Slackware Current	4.14.290	slackware_4_14_pid0.txt	slackware_4_14_pid1.txt	slackware_4_14_pid2.txt
CentOS 7.9	3.10.0-1160.71.1.el7.x86_64.	centos_3_10_pid0.txt	centos_3_10_pid1.txt	centos_3_10_pid2.txt

You can also get all logs here.

• It will be necessary to modify kernel we are compiling for general distributions and Oracle (Another PR).

[MrZammler]

Just some notes in case any other reviewers need it:

1) Cloned this branch first to a dir (e.g. kernel-collector-324) 2) Downloaded the artifact close to my kernel (artifacts-5.15.63-glibc) into the artifacts/ directory. 3) Did a git submodule update --init --recursive 4) make and make tester. 5) Run the tests as root with for i inseq 0 2; do ./kernel/legacy_test --netdata-path artifacts --content --iteration --pid $i --log-path file_pid$i.txt; done. The netdata-path needs to be the path to the artifacts downloaded before.

Artifacts only contains cachestat, so ignored the errors for not finding the others. For it I got Success:

"/home/evas/tmp/kernel-collector-324/artifacts/rnetdata_ebpf_cachestat.5.15.o" : {
    "Test" : "return",
    "Tables" : {
        "cstat_global" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 8},
                       "Type" : 6,
                       "FD" : 5,
                       "Data" : [
                                    { "Iteration" :  0, "Total" : 4, "Filled" : 3, "Zero" : 1 }
                                ]
                      }
        },
        "cstat_ctrl" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 4},
                       "Type" : 2,
                       "FD" : 6,
                       "Data" : [
                                    { "Iteration" : 1, "Total" : 2, "Filled" : 2, "Zero" : 0 }
                                ]
                      }
        },
        "cstat_pid" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 32},
                       "Type" : 5,
                       "FD" : 7,
                       "Data" : [
                                    { "Iteration" :  0, "Total" : 0, "Filled" : 0, "Zero" : 0 }
                                ]
                      }
        },
        "Total tables" : 3
    },
    "Status" :  "Success"
},

My setup: `Linux 6am 5.15.69-gentoo #1 SMP Sun Oct 9 09:59:07 EEST 2022 x86_64 AMD Ryzen 5 4500U with Radeon Graphics AuthenticAMD GNU/Linux`

[stelfrag]

"/home/stelios/library/rnetdata_ebpf_cachestat.5.15.o" : {
    "Test" : "return",
    "Tables" : {
        "cstat_global" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 8},
                       "Type" : 6,
                       "FD" : 5,
                       "Data" : [
                                    { "Iteration" :  0, "Total" : 4, "Filled" : 4, "Zero" : 0 }
                                ]
                      }
        },
        "cstat_ctrl" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 4},
                       "Type" : 2,
                       "FD" : 6,
                       "Data" : [
                                    { "Iteration" : 1, "Total" : 2, "Filled" : 2, "Zero" : 0 }
                                ]
                      }
        },
        "cstat_pid" : {
            "Info" : { "Length" : { "Key" : 4, "Value" : 32},
                       "Type" : 5,
                       "FD" : 7,
                       "Data" : [
                                    { "Iteration" :  0, "Total" : 1, "Filled" : 1, "Zero" : 0 }
                                ]
                      }
        },
        "Total tables" : 3
    },
    "Status" :  "Success"
},

[tkatsoulas]

Approving without reviewing