CI: switch to GitHub Actions

[jrfnl]

CI: switch to GitHub Actions - step 1: sniff stage

This commit:

• Adds a GH Actions workflow for the CI checks which were previously run on Travis in the sniff stage.
  While these aren't 100% CS (= code style) checks, for the badge and workflow display, CS still seemed the most descriptive name.
• Removes that part of the .travis.yml configuration.
• Adds a "Build Status" badge in the Readme to use the results from this particular GH Actions runs.

Notes:
Builds will run on all pushes and on pull requests, with the exception of those just modifying files which are irrelevant to this workflow.

CI: switch to GitHub Actions - step 2: docs stage

This commit:

• Adds a GH Actions workflow for the CI checks which were previously run on Travis in the docs stage.
• Removes that part of the .travis.yml configuration.
• Adds a "Build Status" badge in the Readme to use the results from this particular GH Actions run.

Notes:

• Builds will run on all pushes and pull requests which contain changes to files in the docs folder.
• With the way the setup-ruby action works, gems will be installed in a docs/vendor folder. For that reason, that folder now needs to be excluded from the build.

CI: switch to GitHub Actions - step 3: quick test stage

This commit:

• Adds a GH Actions workflow for the CI checks which were previously run on Travis in the quicktest stage.
• Removes that part of the .travis.yml configuration.

Notes:

1. Previously, this "stage" would run on all push events, except when merging to master or develop.
   The current set-up still does so, with one exception: pushes to develop will now use this quicktest instead of the full test.
   develop is a protected branch anyhow, so all merges will already have had the full test run in the pull request.
   For merges to master, the full Test run will still be used as that will generally mean we're getting ready to tag a release and some extra safety checking before a release is not a bad thing.
2. This also updates the "high" PHP version for the "quicktest" against PHPCS dev-master from PHP 7.4 to PHP 8.0 by using latest which translates to "latest stable PHP release".

CI: switch to GitHub Actions - step 4: test and coverage stage

This commit:

• Adds a GH Actions workflow for the CI checks which were previously run on Travis in the test and coverage stages.
• Removes the, now redundant, .travis.yml configuration.
• Updates the .gitattributes file.
• Updates the "Build Status" badge in the Readme to use the results from the GH Test Actions runs.
• Updates the "Tested on" badge in the Readme to include PHP 8.
• Updates a test file which mentioned Travis.

Notes:

1. Each of these jobs has a needs dependency on the previous job to prevent it from starting if the previous job failed.
   While not 100% necessary, this is just an efficiency tweak, being kind to the free service being offered as we know that if linting fails, the tests will fail etc.
2. The builds in the test and coverage jobs are essentially the same as previously run on Travis, though PHP 8.0 is now fully accounted for and 8.1 is set as nightly.
3. Previously, this "stage" would run on all pull requests events.
   The current set-up still does so, with one addition: pushes to master (merges) will now also use this workflow instead of the quicktest.
   This replaces the full run on tagging a release, meaning that thing will essentially be the same.
4. As there are a couple of jobs which are "allowed to fail" (experimental = true), the build status will unfortunately show as "failed", even though all non-experimental jobs have succeeded.
   This is a known issue in GHA: Please support something like "allow-failure" for a given job actions/runner#2347

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Future scope:

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;
• Automatically update the documentation site;
• etc

For now, this PR just converts the existing checks to GH Actions.