Docutils: Documentation Utilities Code

Downgrade "duplicate ID" message from "serious" to "error".

A duplicate ID is not a critical error where the output will
inevitably contain severe errors but rather "a major issue that should
adressed. If ignored, the output will contain unpredictable errors."
(cf. :PEP:`0258` Error Handling).

Re-use `nodes.document.set_id()` in the XML parser, but
only for registering an existing ID, do not generate IDs.

"include" directive: handle duplicate names and IDs in sub-documents

When the :parser: option is specified, the included file is parsed
into a dummy `<document>` and children are appended.
Let internal bookkeeping attributes reference the main document
equivalents, so that element names and IDs share a namespace
with the including document.

Use UNIX-style (LF) line endings

Use custom TypeAlias `StrPath` for `str`-based file system path vars.

Define TypeAlias for "str or os.PathLike" in docutils.nodes
(as this module does not depend on other docutils modules).
Import and use in all places where ``str | os.PathLike`` is expected.

Align formatting, use f-strings where it helps clarity.

Document two more doctree elements + other documentation updates.

Document `<problematic>` and `<system_message>` doctree elements.

Various more or less related documentation corrections and updates.

Support CSS3 `length units`_.

Add "ch", "rem", "vw", "vh", "vmin", "vmax", and "Q" to the length units
supported by reStructuredText.

Note that some output formats don't support all units.

HTML: CSS3 units are supported by all contemporary browsers.

XML, pseudoxml: Docutils Document Tree elements validate
"any run of letters or a percent sign" as a measure's unit.

LaTeX: the writer emulates non-native units with custom length macros.

ODT: the writer only supports fixed length units (by converting to "cm").
Support for the unit "Q" is added.

man: the writer ignores length specifications.

Fixes feature-request #57.

LaTeX writers update.

Deprecate `writers.latex2e.SortableDict`.

Remove `writers.latex2e.Table.get_caption` (obsolete).

Test and fix local ToC without global ToC.
Package "minitoc" needs a ``\faketableofcontents`` in this case so that a
".toc" file is generated.
Move the conditional insertion code in `depart_document()` before the
conversion of `self.requirements` from a dict to a list, so that the
test works again as intended.

Minor documentation fixes.

HTML5: Fix display of inline images in section headings.

Add selectors for an image inside h1 ... h6 to the rule for inline images.

Update/fix "standard" functional test sample.

Fix a factual error
(footnotes must be indented at least 1 space, not 3).

Reword HTML-centric descriptions -- we use the sample for several
writer tests.

Revert change of expected pseudo-xml sample extension from "txt" to "rst"
(side-effect of the switch to "rst" extension for rst sources).