More options for building/installing documentation to ease working on it

At the moment working on the documentation is a bit tedious. Checking my changes before committing involves building and installing, check my changes, fix/improve, build and install again, … you get the idea.

**Status quo:**

* All plots are generated from scratch every time. That is quite time consuming and for the most part unnecessary when I'm just fixing links, typos, layout, etc. Sure, `--parallel` does help but still wasted time.
* No apparent way of installing just the documentation. Not really necessary to install everything (gmt executable, library, development headers and built-in data) every time. Not as time consuming as building but still wasted time.

**Desired feature:**

* Have options to build
* just the `.rst`
* just the plots
* both (default).
* Have options to install
* just the documentation
* everything (default)

**Benefits:**

* Would ease/accelerate working on the documentation.

_See https://github.com/GenericMappingTools/gmt/pull/7222#discussion_r1087558281 for the original conversation._