Add use case: Issue Index Markdown generator

Description

gen_issue_index.py queries the API for a given project ID, collects all labels and issues and generates a grouped by label index in Markdown format, which gets dumped into index.md, and added as commit to the repository itself.

Example in https://gitlab.com/dnsmichi/api-playground/-/blob/main/index.md

References

Inspired by this HN Comment: https://news.ycombinator.com/item?id=32153549

We tried using the Wiki of a gitlab project in my company to gather some knowledge in a single place, but it was too "hard" to go to a page and edit it. So we moved the content in the repo. Again, we had to search if our content was already written or not, and we were spending time trying to find files and words instead of writing shorts texts.

So we moved our content on the gitlab issues of the repo.

Issues have the same markdown support, we can use ctrl+v to insert screenshots, we can add tags to the issues, we can assign people to relevant issues, we can comment with new things to do/add, and we can use the search bar to find relevant content. All our "knowledge doc" is stored insides the issues of an empty project named "doc".

We were just thinking about something to replace an older system not used anymore, and since it's been 2 years I think I can say that issues really work well for us (small team, 6 devs, some of us write more issues than others).