Move docs to a separate directory

doc/url-params.md

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+# URL parameters
+
+gamja settings can be overridden using URL query parameters:
+
+- `server`: path or URL to the WebSocket server
+- `nick`: nickname
+- `channels`: comma-separated list of channels to join (`#` needs to be escaped)
+- `open`: [IRC URL] to open
+- `debug`: if set to 1, debug mode is enabled
+
+Alternatively, the channels can be set with the URL fragment (ie, by just
+appending the channel name to the gamja URL).
+
+[IRC URL]: https://datatracker.ietf.org/doc/html/draft-butcher-irc-url-04