Documentation Updates

[cmsirbu]

What's changed

• Moved existing documentation into a new structure to prepare for the mkdocs rendered docsite.
• Edited and improved the visuals in various places using new syntax from the Material for Mkdocs theme.
• Added configuration for building and hosting the docsite on RTD.
• Replaced the project README with a standardized one. Note: the README will have certain links/images broken until the PR is merged into develop.

TODO

Some sections throughout the files listed below require input (or removal if not applicable to this plugin). All Developer Note comments should be resolved and removed.

• [ ] user/app_overview.md
• [ ] user/app_use_cases.md
• [ ] user/app_getting_started.md
• [ ] user/faq.md
• [ ] user/external_interactions.md
• [ ] admin/install.md - review (most old README content) ported here
• [ ] admin/upgrade.md
• [ ] admin/uninstall.md
• [ ] admin/compatiblity_matrix.md
• [ ] admin/release_notes - I think the CHANGELOG content should be linking to the docs section release notes, but will leave this decision up to you.
• [ ] dev/extending.md - review
• [ ] dev/contributing.md
• [ ] dev/dev_environment.md - boilerplate guide based on cookie template (old GETTING_STARTED guide). A sync to the cookie dev environment would be beneficial to ensure instructions are accurate.
• [ ] dev/code_reference - check examples provided and add any module/package auto-generated docs that are relevant
• [ ] dev/arch_decision.md
• [ ] README.md - review its contents!

Other issues:

• [ ] Added test to check that RTD used docs/requirements.txt are in sync with dependencies in pyproject.toml - needs the following item fixed first, might need to remove it?
• [ ] Couldn't add docs dependencies (see docs/requirements.txt for exact versions) to poetry environment for development because of Jinja2 conflict with Nautobot 1.2.

And because nautobot (1.2.11) depends on Jinja2 (>=2.11.3,<2.12.0)
   and mkdocs-material (8.4.2) depends on jinja2 (>=3.0.2), mkdocs-material (8.4.2) is incompatible with nautobot (>=1.2.1,<1.3.0).