Converting gap-analysis doc to work with issues

[r12a]

We are converting the template for the gap-analysis documents so that most of the content can be written/edited in github issues. This makes editing easier, but also establishes a GH issue thread that can bring together discussion, comments, and suggested text for developing the text on a given topic.

The document at https://w3c.github.io/alreq/gap-analysis/ will automatically retrieve the appropriate content from the GH issues list. Changes to the latter will be immediately visible in the document if you refresh your browser page.

By reducing the complexity involved in maintaining the document in HTML and using the GH pull request process, it is hoped that this will make it easier for people to contribute to the document, as well as providing a handy list of sections in the issue list when filters are applied.

As we convert the document to the template, you will see a flurry of issues added to this repository. Apologies in advance for that.

[r12a]

This has been done.

From now on, if we want to change the content of the gap-analysis doc, it's a question of raising a new issue (if it's a new topic), or editing (or proposing edits for) an existing issue. (Or closing the issue, which will remove the content from the document.)