Update version numbers etc for release 1.26.3.

[Partial release 1.26.2 was deleted after a few minutes on pypi.org because was
faulty.]

tests/: update expectations for latest mupdf master.

Set release date.

Documentation Updates

Insert cautionary comment to method Document.new_page.

Update the API of package PyMuPDF4LLM

Update version numbers and dates for release.

Update version numbers and dates for release.

Update version numbers and dates for release.

tests/test_memory.py: avoid spurious failures on non-linux.

Non-Linux machines have unreliable RSS values on Github.

scripts/gh_release.py: fixed to match lower-case sdist and wheel names.

setup.py: incremented default mupdf version to 1.24.11.