Move documentation to Gitbook

[unfulvio-godaddy]

Summary

This PR moves the bulk of the documentation from the README.md file over to /docs which can be handled by Gitbook.

This is a preview of how it will look like: https://wp-mock.gitbook.io/documentation/general/introduction but the source of truth will be this PR - the current copy on GitBook is not synced with Git currently and incomplete. I think we need to initiate a diff from this repo, sync won't be bidirectional.

Note that the sources of documentation will always be in this repository under /docs -- if some day we switch away from GitBook the documentation will still be readable - as a matter of fact can be read from within the repository. Editing the docs will also be easy through regular PRs.

For a preview of the updated README.md: https://github.com/10up/wp_mock/tree/chore/move-docs-to-gitbook

Closes: #199

Details

After this is merged we need to make sure the docs will be properly synced to Gitbook from the markdown files in /docs.

The CREDITS.md file is removed - the list of contributors is automatically generated by GitHub and furthermore we can make attributions in the generated changelogs. There's no need to manually maintain a static file of contributors.

[coveralls]

Coverage: 41.94%. Remained the same when pulling 6b8eaaa on chore/move-docs-to-gitbook into 4a46042 on trunk.

[jeffpaul]

I've not tested that these changes actually work as desired with Gitbook, but they otherwise look to properly move the docs correctly