Make Doxygen Member Groups stand out on Read the Docs

[claremacrae]

Where classes have lots of similar methods, I've used Doxygen Member Groups to group methods together - for example, in Approvals.h.

Currently, the names of the member groups don't stand out at all on Read the Docs, because they are indented to the same level as their contents.

This is what the above linked code example looks like:

[claremacrae]

The names of member groups are now more visible:

Closing the ticket.

[claremacrae]

The screenshots above are from this page:

https://approvaltestscpp.readthedocs.io/en/latest/api/approving.html