Reorganize Solr setup docs

Sphinx doesn't support "inner pages" like the Solr setup page was trying
to be, it breaks Sphinx's navigation.

Move the single-core Solr setup docs into the source install docs,
inline.

And an appendices chapter to the docs, and move the multi-core Solr docs
into it.

Also add new docs for setting up a second Solr core when using
multi-core.

Also tidy up some Sphinx heading styles.