Documentation: constrain alabaster package to older versions

The 'alabaster' theme dropped support for Sphinx < v3.4:

    0.7.14 – 2024-01-08

    * Dropped support for Python 3.8 and earlier.
    * Dropped support for Sphinx 3.3 and earlier.
    [...]

(Source: https://alabaster.readthedocs.io/en/latest/changelog.html)

This manifests as an error when running 'make htmldocs' in a virtualenv
constructed from Documentation/sphinx/requirements.txt:

    Sphinx version error:
    The alabaster extension used by this project needs at least Sphinx v3.4; it therefore cannot be built with this version.

Raising the Sphinx version is not really a good option at this point,
since 3.x through 6.x have horrible performance regressions (7.x still
does, but not quite as bad).

Instead, constrain the 'alabaster' package to versions that still support
Sphinx 2.4.4.
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>
Link: https://lore.kernel.org/r/20240110104646.3647600-1-vegard.nossum@oracle.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/sphinx/requirements.txt

 # jinja2>=3.1 is not compatible with Sphinx<4.0
 jinja2<3.1
+# alabaster>=0.7.14 is not compatible with Sphinx<=3.3
+alabaster<0.7.14
 Sphinx==2.4.4