doc build instructions omit the sphinx theme

[neilkatin]

I attempted to follow the documentation build instructions, but it failed for me because the required sphinx theme (sphinx-rtd-theme) was not installed.

This is a one line change; PR will follow.

[RogerSelwyn]

You need to install the dev dependencies as defined in pyproject.toml

[neilkatin]

Perhaps, but the docs don't mention that; they just say to install sphinx.

[RogerSelwyn]

Fair enough