Add Matrix .well-known files

docs/static/_headers

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+/.well-known/matrix/*
+  Access-Control-Allow-Origin: *
+  Content-Type: application/json

docs/static/client

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+{"m.homeserver":{"base_url": "https://matrix.continuwuity.org"},"org.matrix.msc3575.proxy":{"url": "https://matrix.continuwuity.org"}}

docs/static/server

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+{"m.server":"matrix.continuwuity.org:443"}