feat: add well-known matrix file

[braaar]

This is necessary for receiving traffic to our matrix server which will be hosted on matrix.bjerk.io but is identified publicly with the :bjerk.io address

See the delegation docs for more info

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 6211505):

• https://dev-bjerk-io--pr707-matrix-well-known-8aa6og3m.web.app
• https://bjerk-io--pr707-matrix-well-known-2h3nsa5n.web.app

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[braaar]

Sound good!