Keep techlibs folder hierarchy.
techlibs/* and passes/* groups are now nested under index_techlibs and index_passes respectively, with most (all?) of the passes/* pages getting proper headings, as well as backends/frontends/kernel. `index_passes_techmap` also references `index_techlibs`.
Split command reference toc in twain, one with maxdepth=2 and one with maxdepth=3, since passes and techlibs now have an extra level of nesting.
Move the `cmd_ref` link to the command reference, instead of top of the page.
Remove `index_internal` and `index_other` from the toc, and mark the pages as orphan. Internal commands get a note callout after the command reference toc (although this doesn't work for the pdf build), while other commands are linked in the warning for missing `source_location` (since that *should* be the only time when there are any commands in the "unknown" group).
Update autodoc extension versions, and mark the directives extension as not `parallel_read_safe` (it might be, but I'm not sure about how the xref lookups work if it is parallel so better to be safe).
Calling :cmd:title:`<cmd>` will generate a cross reference to `<cmd>`, but unlike :cmd:ref: which displays a literal block and puts the title (short_help) in the hovertext (the title field of an a-ref), :cmd:title: will display "<cmd> - <short_help>" as plain text.
Thus replacing the previous use case of referring to :doc:`cmd/<cmd>`.
Also refactor util py scripts to have more descriptive names.
