doc: update `README` to include `CI` badges and `contributors` avatars

[baimamboukar]

This pull request comes with changes to make the project's README more presentable. It introduces the following changes:

• add workflows and license badges in README
• add contributors avatars
• add github header/image for the repository

[gnprice]

These aren't things we need in our README, so closing this.

This PR also includes a number of new files that aren't related to the description. See our guide about keeping a clean Git history: https://zulip.readthedocs.io/en/latest/contributing/commit-discipline.html

If you're interested in contributing to Zulip, see our guide here: https://github.com/zulip/zulip-flutter#contributing and in particular this guide (linked from there) about picking an issue to work on: https://zulip.readthedocs.io/en/latest/contributing/contributing.html#picking-an-issue-to-work-on