Restructure Readthedocs documentation

[vishalvvr]

Pbench docs(Readthedocs) need a rework, so this is the first step toward documentation efforts.
This is just a structural change, this commit does not intend to make content-level changes.

Changes:

• New directory structure.
• Copy existing Contributing, Server API doc, getting started, user guide, and system design files to new dir
• Add support for markdown files(earlier it was only reStructuredText).

Demo link:
https://pb-readthedocs.readthedocs.io

PBENCH-1152

[dbutenhof]

• black --check .
  would reformat docs/conf.py

I'm not sure we should be black checking anything under docs, so one option would be to add this to the exclusion list (pyproject.toml)

[dbutenhof]

You need to fix the black issue, either by formatting the conf.py or (I think preferably) excluding it and probably the entire docs tree.

[vishalvvr]

You need to fix the black issue, either by formatting the conf.py or (I think preferably) excluding it and probably the entire docs tree.

Fixed

[dbutenhof]

I would have slightly preferred changing the configuration to exclude the conf.py file (and probably everything under docs/), but I guess this works and I suppose it's unlikely there will be any more. So let's just get this in and move on.