WIP: Makefile `servedocs` target with live reload; reorder subsections in "Config" and "Colors & Appearance"

[ernstki]

Marked "WIP" since it stacks on the commits from #6412

Assuming that gets merged, this PR will add:

• a make servedocs target that live-reloads, with the mkdocs --dirtyreload option, which reduces build time from ~30 seconds to just seconds
  • this required removing strict: true from docs/mkdocs-base.yml, but adding --strict to the make build target (not sure if this will break CI, though)
• better subsection structure for "Config" and "Colors & Appearance," so that similar config settings are grouped together
• many small stylistic fixes elsewhere, including the index

[wez]

Please separate out the makefile and doc build changes from the doc contents changes, as blending them together makes it difficult to review!

[ernstki]

Please separate out the makefile and doc build changes from the doc contents changes, as blending them together makes it difficult to review!

Gotcha, that's a valid point, and will do. Thanks for coaching me on it!