Add dark theme to docs and Blog

[Andre601]

Makes the following changes:

• Moves mkdocs-user-docs.yml into the docs/ directory
• Renames docs/blog.yml to docs/mkdocs-blog.yml for better consistency with file names used.
• Updated bin/rtd-docs to use new file name and locations.
• Added a dark theme and theme toggle to documentation.

See #13294 for context and any extra info.

[miketheman]

Looks good, thanks!

[miketheman]

@Andre601 looks like one missing ref:

warehouse/bin/user-docs

Line 13 in 9458ec9

mkdocs build -f mkdocs-user-docs.yml

[miketheman]

Thanks @Andre601 !