Expand GH Actions

[jrfnl]

From #239:

In the future additional workflows/jobs could be added to, for instance,

• Check for broken links in the documentation site every month.
• Check the markdown files for style, spelling and broken links. GH Actions: add linting and QA checks for markdown documents #312
• Check Yaml files for style and formating. GH Actions: add Yamllint to QA basics #313
• Automatically update the documentation site. GH Pages/GH Actions: generate the documentation instead of committing it #419
• Run the tests with non-required PHP extensions (iconv, mbstring) disabled. GH Actions: expand the test matrix #316