chore(docs): Rework based on community feedback

[niladrih]

Changes in this PR: 1st commit:

1. Changed the order of appearance of the 'RKE/Rancher 2.x' section and the 'RancherOS' section based on their popularity in the community.
2. Added a 'Note' to state the incompatibility of cStor CSI with RancherOS.
3. Removed sections 'iSCSI services on RHEL or Ubuntu 16.04' and 'iSCSI services on Ubuntu 18.04 or CentOS 7.6'. Replaced them with the section 'iSCSI services on RKE/Rancher 2.x'.
4. Changed the grammar and phrasing on some of the instructions in the RancherOS section.
5. Added 'sudo' to all of the commands in the 'iSCSI services on RKE/Rancher 2.x' section which commonly require superuser privileges.

2nd commit:

1. Removed modprobe iscsi_tcp and echo iscsi_tcp > /etc/modules-load.d/iscsi-tcp.conf because once open-iscsi packages or iscsi-initiator-utils packages are installed an entry for the iscsi_tcp module is automatically created in /lib/modules-load.d. Also, the module is loaded before the service is activated for the first time -- can be confirmed from dmesg logs.

3rd commit:

1. Changed <a class="anchor" aria-hidden="true" id="rancher-ubuntu-centos"> to <a class="anchor" aria-hidden="true" id="rancher-rke">
2. Changed <a class="anchor" aria-hidden="true" id="rancher"> to <a class="anchor" aria-hidden="true" id="rancher-rancheros">
3. Rebased

Signed-off-by: Niladri Halder niladri.halder@mayadata.io

[netlify[bot]]

Deploy preview for openebs-docs-staging ready!

Built with commit 23e2d6246455f085d163c2a890302833856012d6

https://deploy-preview-882--openebs-docs-staging.netlify.app

[kmova]

@niladrih -- looks good. Can you rebase the PR to pass the travis please.

[niladrih]

@kmova -- Done.