Documentation theme update

[Chris Johns]

Hi,

I have a patch for rtems-docs.git to move us to the pip installed
sphinx-rtd-theme removing the custom theme based on sphinx-rtd-theme we
currently use.

The ticket is #4994.

The patch is over 4M in size as it deletes common/sphinx_rtd_theme_rtems. YOu
can download it from:

 https://ftp.rtems.org/pub/rtems/people/chrisj/0001-sphinx-Use-the-pip-installed-sphinx-rtd-theme.patch

What I am not sure about is how old Sphinx can be to build the documentation. My
versions are:

 Sphinx                        7.2.6
 sphinx-rtd-theme              2.0.0
 sphinxcontrib-applehelp       1.0.7
 sphinxcontrib-bibtex          2.6.1
 sphinxcontrib-devhelp         1.0.5
 sphinxcontrib-htmlhelp        2.0.4
 sphinxcontrib-jquery          4.1
 sphinxcontrib-jsmath          1.0.1
 sphinxcontrib-qthelp          1.0.6
 sphinxcontrib-serializinghtml 1.1.9

Is it OK to push?

Chris