A GH Pages site needs a Gemfile to be able to generate and to allow for testing locally.

It may be that in the past this was not a hard requirement, but either way, it is good practice to have the file in place.

Includes adding the `lock` file to `.gitignore` and both the `Gemfile` and the `lock` file to the list of files which should not be copied over into the live website.

... to stop Jekyll complaining about the favicon.ico file missing.

Note: I've not added any code to the page header for the favicon as all modern browsers will pick up on it automatically anyway.

At this moment, there is only one link to the API documentation and it is hidden away on the first page of the "Documentation" page where it can be easily overlooked.

This adds a new menu item in the main menu at the top of the page linking to the API Docs for improved discoverability.

Code blocks and inline code snippets were no longer parsed correctly.

Additionally, in the previous generated site, the syntax highlighting of code was already broken.

This commit fixes both.

Includes:
* Adjusting the script which adjusts the files from the `master` branch and pulls them into the `gh-pages` branch.
    The string replacement for PHP code blocks is no longer needed and actually detrimental.
* Removing most `{% highlight ... %}` liquid syntax in favour of markdown fenced code blocks.

During the generation of the site, select information from the GitHub API is available in the `site.github` object.

Using that information will allow for automatically updating "frequently" changing - and easily forgotten - content and will lower the maintenance burden.

This commit:
* Enables the use of the GH API via the `jekyll-github-metadata` plugin.
* Ensure that this will work when testing the site locally by adding a `repository` key to the config.
* Adds the retrieved data cache file to the `.gitignore` and the site building `exclude` list.
* Use URLs from `site.github` to refer to various parts of the repository.
    This will automatically update the URLs in the website for the move to the WordPress organisation.
* Use the `site.github.latest_release.tag_name` key to automatically update the version number of the last release in all the relevant places whenever the site is regenerated.
* Automate the generation of the "Previous versions" list on the Download page based on the information from the GH API.

Ref: https://github.com/jekyll/github-metadata/blob/master/docs/site.github.md

Also:
* The `gems` key is deprecated. Replaced by `plugins`.

...  in templates used throughout the site and create those absolute URLs the Jekyll way.

This will create the URLs as absolute URLs for the online version of the website, while for local testing relative URLs will be used.

This commit:
* Adds a custom `site.requests` array to the config file for information used in various places in the site.
* And then updates the site to use those variables instead.

This will make it easier to update this information througout the site in one go.

* Blank line between frontmatter and the page title.
* Update the URL for Composer.

…he site

Any files in the root directory and in directories which don't start with a `_`, will normally be copied into the site and depending on settings be processed for variables and markdown parsing.

The `exclude` key allows to ignore select files during site generation.

This commit:
* Expands the list with:
    - the Jekyll defaults as the list overwrites the defaults;
    - various files already in the branch, which should not be copied to the site;
    - a few common backup file types which may be present while working on the site locally.
* Adjusts the format of the key for readability.

Ref: https://github.com/jekyll/jekyll/blob/master/lib/site_template/_config.yml#L37-L55

Sync the files in the `docs` subdirectory with the changes made in the `master` branch.

... which got removed by the APIDocs regeneration.