GitLab

* Replaces sub tags for python API docs

* Changes string to pathlib

* Adds support for sup html tags

* Corrects function description

* Proof of concept matching current behaviour with Breathe

* Reorganise C++ API docs to work without autosummary

* Revert to Sphinx-native search

* Move remaining pip deps to conda

* Remove unnecessary lunrsearch templates

* Remove lunrsearch from cmake

* Tidy up layout of API docs

* Get code blocks to work with Breathe

Breathe doesn't seem to support the Doxygen <preformatted> tag.
It does support the <code> tag, but better yet it supports using
<verbatim> tags to embed rst into docstrings. This commit changes
all <pre> tags to use RST verbatim, and updates the Python
documentation machinery to support it too.

* Clarified some language

* Have doxygen exclude undocumented classes and have sphinx fail on warnings for C++ API docs

* List custom forces and integrators last

* Add breathe to documentation CI

* Typo

* Fix link to custom forces in extras.rst

* Have Breathe process only public classes

* Strip OpenMM:: prefix from rst files to avoid colons in links

* Remove existing private classes from EXCLUDE_SYMBOLS

* Add comment to C++ cmake describing why we promote warnings to errors

* Revise documentation build instructions for new dependencies