CI testing of sphinx documentation

We should be enforcing warning-less sphinx build. Practice shows rST files tend to fall behind with code changes.

Steps:

• fix remaining errors and warnings in sphinx_docs target after latest Doc Day changes are all merged in;
• add sphinx (and RTD theme) to either base Docker images or the TPL build (I think version differences are less severe there than in doxygen, but I may be horribly wrong);
• add a CTest test with a custom script that checks the output of make sphinx_docs command (this is needed because the command exits with 0 even in presence of errors). Adding -w or -W to sphinx-build might be helpful if BLT allows.
• add a (non-terminating) Stage 1 CI check that runs said test, or add to an existing check

Start with the sphinx_docs target and inspect how the current documentation build reports warnings and errors. Then review the CTest integration and Stage 1 CI checks, along with the base Docker images or TPL build configuration. Done means a warning-free Sphinx build is tested and the check runs in CI without terminating the stage.