Generate man pages

[zuiderkwast]

A Makefile provides 'make' and 'make install' to generate and install man pages.

Man pages are built for the binaries (valkey-cli, etc. prefixed by "valkey-", section 1), Config (valkey.conf, section 4), Valkey commands (section 3valkey), documentation overview page ("valkey", section 7) and tutorials and everything else (prefixed by "valkey-", section 7).

Scripts in Python are used for preprocessing and pandoc is used to convert markdown to man pages, stored under utils/.

A target 'make html' is also provided, to build HTML pages to use locally. This is also using pandoc.

The documentation pages for valkey-cli, valkey-benchmark and valkey-sentinel are expanded with usage and options to match what's expected from a man page for a program. A page for valkey-server is added. (We didn't have one.)

README.md is update with info on how to build and install man pages. Other info regarding the doc repo is slightly updated.

Fixes #69.

Reviewers: I suggest checking out my branch and trying make -j10 VALKEY_ROOT=path/to/valkey and sudo make install. Then man valkey to start with.

[zuiderkwast]

[zuiderkwast]

@Conan-Kudo @guillemj FYI