Update documentation links for new domain

[citizen428]

This PR update relevant links (e.g. to installation instructions) in the main README to point to https://element-hq.github.io/dendrite instead of https://matrix-org.github.io/dendrite.

Pull Request Checklist

• I have added Go unit tests or Complement integration tests for this PR or I have justified why this PR doesn't need tests
• Pull request includes a sign off below or I have already signed off privately

Signed-off-by: Michael Kohl <[email protected]>

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[citizen428]

All changes below here were automatically done by my editor which is configured to remove trailing whitespace. I can undo these specific changes if requested.