Docs: merge sections that address catalog customization

[Kathy Lussier]

The current docs have three chapters that address OPAC customization:

http://docs.evergreen-ils.org/2.12/_designing_your_catalog.html

and

http://docs.evergreen-ils.org/2.12/_tpac_configuration_and_customization.html

and http://docs.evergreen-ils.org/2.12/_creating_a_new_skin_the_bare_minimum.html

The TPAC configuration and customization chapter and creating a new skin chapters came first, but the designing your catalog section was added separately as part of an effort to move the contents of the Evergreen in Action book to the official docs.

At this point, I think the three could be merged. I intend to start with the Designing Your Catalog section as the base and then merge in whatever is missing from the others.

[Andrea Neiman (aneiman)]

In Antora docs, these are together in a section.

Also, since these docs are related to TPAC, I'm making a note for the next DIG meeting to discuss removing TPAC docs from master going forward.