Read the Docs stopped installing spinx-rtd-theme by default, which
broke documentation builds. This commit fixes that by pinning the
required dependencies.

See: https://blog.readthedocs.com/defaulting-latest-build-tools/

The Sphinx Emacs Lisp extension assumed all \[command] key sequence
substitutions in documentation strings are preceded by a \<mapvar>
sequence specifying the keymap, unlike substitute-command-keys which is
used when accessing docstrings within Emacs.

With this commit the actual substitute-command-keys function is used
when building docstringdb.json, instead of the Python reimplementation.
The drawback is that docstringdb.json now contains partial Sphinx
markup.