build(docs): update Starter Pack and add passthrough to docs Makefile

[medubelko]

Add aliases and passthrough Make targets provided by the docs project. With this, you can run make at the root of the project and access docs commands. The behaviour is:

• docs safely calls html
• docs-auto safely calls serve
• docs-clean safely calls clean
• docs-install calls vale-install and then calls root setup-docs
• docs-* passes through to all other doc targets

The goal is to keep the docs build nearly unedited. For this to work as of Starter Pack 1.3.1, the VALEDIR var in the docs Makefile must be changed to a conditional assignment (?=).

---

• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.

[medubelko]

The workflows are failing:

WARNING: the sphinx_toolbox.latex extension does not declare if it is safe for parallel reading, assuming it isn't - please ask the extension author to check and make it explicit

This wasn't a problem before because we allowed warnings, but the Starter Pack has fail-on-warning for both Sphinx builders. I'd rather not override it if we can avoid it. I'll look into whether we're using sphinx-toolbox in our projects aside from the collapse element. That one has a drop-in replacement.