This removes most of the code used for polylith/API mode.
This removes the `/api` internal endpoints entirely.
Binary size change roughly 5%:
```
51437560 Feb 13 10:15 dendrite-monolith-server # old
48759008 Feb 13 10:15 dendrite-monolith-server # new
```
I have added/copied a landing page like Synpase does.
Recently I have installed Dendrite and was wondering why it´s not
working. After some troubleshooting I figured out there is no landing
page like synpase has, so the Server was running just fine.
Hopefuly this PR can fix this problem and may help other users who run
into this issue.
I have not written any unit tests, because it´s just a simple landing
page with a redirect to a static site.
### Pull Request Checklist
<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->
* [x] I have added Go unit tests or [Complement integration
tests](https://github.com/matrix-org/complement) for this PR _or_ I have
justified why this PR doesn't need tests
* [x] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately
Signed-off-by: `Lukas Huida<lukas@leucali.net>`
Co-authored-by: Till Faelligen <2353100+S7evinK@users.noreply.github.com>
