Our `devtest` page contains some good UI examples which should be taken
as example when working on Gitea UI. Mention it in our frontend
contributing guidelines to for new contributors to know about it.
This PR cleans up the docs in a way to make them simpler to ingest by
our [docs repo](https://gitea.com/gitea/gitea-docusaurus).
1. It includes all of the sed invocations our ingestion did, removing
the need to do it at build time.
2. It replaces the shortcode variable replacement method with
`@variable@` style, simply for easier sed invocations when required.
3. It removes unused files and moves the docs up a level as cleanup.
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Signed-off-by: jolheiser <john.olheiser@gmail.com>
