docs: Add guide for push notifications (#2244)

Adds a page for push notifications under /guides. The main thing it covers are how push works in Matrix, UnifiedPush usage and selfhosting, and some troubleshooting of common push issues.

I took the liberty to include various aspects of push not pertaining to the server itself, because it was commonly asked in our chatrooms and are quite important for all matrix users. Kind of like livekit.

Closes #2136

Reviewed-on: https://forgejo.ellis.link/continuwuation/continuwuity/pulls/2244
Reviewed-by: Jade Ellis <jade@ellis.link>
Reviewed-by: Henry-Hiles <henry@henryhiles.com>
Reviewed-by: lveneris <tyler@kludgecs.com>

docs/deploying.mdx

@@ -99,7 +99,7 @@ After Continuwuity is up and running, consider the following:
 
 - To set up Audio/Video communication, see the [**Calls**](./calls.mdx) page.
 
-- See the [**Authentication chapter**] for various ways to access to your homeserver, such as logging in with an external OIDC provider.
+- See the [**Authentication chapter**](./authentication.mdx) for various ways to access to your homeserver, such as logging in with an external OIDC provider.
 
 - Consult the [**Maintenance**](./maintenance.mdx) page for guidance on maintaining your homeserver.