Docs: Grouping changes

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).

docs/util/custom_directives.py

@@ -735,4 +735,7 @@ def setup(app: Sphinx):
 
     app.add_role('autoref', autoref)
     
-    return {'version': '0.2'}
+    return {
+        'version': '0.3',    
+        'parallel_read_safe': False,
+    }