Implement PDF builder based on GitHub Actions

[hobu]

Migrate our building infrastructure from Travis to GitHub Actions. Here's a list of improvements that come as part of this activity:

• Build fails if there are tabs in the RST
• PDF is uploaded as an artifact to each build
• Spell checking with misspelled words added as artifact to each build
• Uses osgeo/proj-docs Docker container, which is more up-to-date (used by GDAL, PDAL, and PROJ)

[hobu]

An example run can be seen at https://github.com/hobu/LAS/actions/runs/263812769

[esilvia]

Does this "action" trigger automatically every time there's a commit on the repo, or does it have to be manually triggered? I see advantages to both, of course.

[hobu]

It is possible to do both