Use sphinx 4.0+

[krashish8]

Problem
Recent build fails for Check Links: https://github.com/pgRouting/vrprouting/runs/4207900423?check_suite_focus=true#step:11:16

WARNING: while setting up extension sphinx.addnodes: node class 'meta' is already registered, its visitors will be overridden
...
build finished with problems, 1 warning.

It is a warning, but warnings are treated as errors, because we use -W flag in sphinx command.

With reference to sphinx-doc/sphinx#9841, this is probably because during pip install, a higher version of docutils is used. So, either we need to restrict docutils to 0.17, or upgrade Sphinx.

As pgRouting uses Sphinx>4.0.0, we can also use it. (pgRouting/pgrouting#2178)

Improvement
Use Sphinx version 4.0 or greater.

This will also imply to use a more resent version of bootstrap