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
• admin/upgrade.md
• admin/uninstall.md
• admin/compatiblity_matrix.md
• admin/release_notes
• 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/design.md - migrated file with edits and formatting, to be reviewed
• dev/code_reference - check examples provided and add any module/package auto-generated docs that are relevant
• dev/arch_decision.md
• README.md

Other issues:

• Couldn't add docs dependencies (see docs/requirements.txt for exact versions) to poetry environment for development because of Python 3.6 and 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).

For reference this is what should be added:

poetry add --dev mkdocs="1.3.1"
poetry add --dev mkdocs-material="8.4.2"
poetry add --dev mkdocs-version-annotations="1.0.0"
poetry add --dev mkdocstrings="0.19"
poetry add --dev mkdocstrings-python="0.7.1"

[bryanculver]

Build blocked by #185

[itdependsnetworks]

Replaced with af87798