Added Landing Page (#2885)

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
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* [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>

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