[docs] [MAJOR] Re-arrange, add a few missing pages to be filled in

[ix5]

Bit of a massive PR but these changes have been resting on my hard drive for way too long.

Changes can be seen live at isso-comments.github.io. Redirects should work fine there (but will need custom nginx rules, when posativ finds some time to upload new docs).

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docs: Re-arrange contents, add placeholders

This is large change that sorts docs pages into a different hierarchy.

Several docs are placeholders for now, to be filled with content later.

Adjust cross-references to new locations, fix inline toctree links

Also: docs: conf: Add redirects mapping

Add a mapping of moved pages for which the reredirects sphinx extension will create 301/JS redirects for.

docs: Annotate missing/incomplete pages with TODOs

And invite people to contribute while showing them how.

docs: conf: Fix favicon path, future-proof toc doc

Makefile: Add css target for docs

docs: css: Adjust fonts, colors, margins, remove >h2

docs: site.scss: Add admonition styling

These are little warnings or notes, presented as boxes.

docs: Add redirects extension

This extension, imported from the sphinx_reredirects package, creates 301/JS redirects for moved pages.

Also: docs: _ext: Emit dirhtml source files

docs: Add hidden theme-testing page

To quickly demonstrate and verify a few features of the _isso theme, such as warning labels.

docs: Remove theme cruft, update styles

docs: Use standard sphinx table-of-content

Rather than a hardcoded one in the sidebar.

docs: conf,theme.conf: More sensible defaults

Needed for later changes which utilize standard sphinx features rather than Isso's hacked-together hardcoded ways.