Fix broken links in the Synapse documentation. (#14744)

* Fix stale external links

* Fix some internal links

* Fix URLs without trailing / where needed

* Fix more links

* Newsfile

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

* Reapply docs/openid.md fix after conflict

Signed-off-by: Olivier Wilkinson (reivilibre) <oliverw@matrix.org>

docs/usage/administration/admin_api/README.md

@@ -7,7 +7,7 @@ server admin. (Note that a server admin is distinct from a room admin.)
 
 An existing user can be marked as a server admin by updating the database directly.
 
-Check your [database settings](config_documentation.md#database) in the configuration file, connect to the correct database using either `psql [database name]` (if using PostgreSQL) or `sqlite3 path/to/your/database.db` (if using SQLite) and elevate the user `@foo:bar.com` to administrator.
+Check your [database settings](../../configuration/config_documentation.md#database) in the configuration file, connect to the correct database using either `psql [database name]` (if using PostgreSQL) or `sqlite3 path/to/your/database.db` (if using SQLite) and elevate the user `@foo:bar.com` to administrator.
 ```sql
 UPDATE users SET admin = 1 WHERE name = '@foo:bar.com';
 ```
@@ -32,10 +32,10 @@ curl --header "Authorization: Bearer <access_token>" <the_rest_of_your_API_reque
 ```
 
 For example, suppose we want to
-[query the account](user_admin_api.md#query-user-account) of the user
+[query the account](../../../admin_api/user_admin_api.md#query-user-account) of the user
 `@foo:bar.com`. We need an admin access token (e.g.
 `syt_AjfVef2_L33JNpafeif_0feKJfeaf0CQpoZk`), and we need to know which port
-Synapse's [`client` listener](config_documentation.md#listeners) is listening
+Synapse's [`client` listener](../../configuration/config_documentation.md#listeners) is listening
 on (e.g. `8008`). Then we can use the following command to request the account
 information from the Admin API.
 

docs/usage/administration/admin_api/federation.md

@@ -81,7 +81,7 @@ The following fields are returned in the JSON response body:
   - `failure_ts` - nullable integer - The first time Synapse tried and failed to reach the
     remote server, in ms. This is `null` if communication with the remote server has never failed.
   - `last_successful_stream_ordering` - nullable integer - The stream ordering of the most
-    recent successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
+    recent successfully-sent [PDU](../understanding_synapse_through_grafana_graphs.md#federation)
     to this destination, or `null` if this information has not been tracked yet.
 - `next_token`: string representing a positive integer - Indication for pagination. See above.
 - `total` - integer - Total number of destinations.
@@ -174,7 +174,7 @@ The following fields are returned in the JSON response body:
   Room objects contain the following fields:
   - `room_id` - string - The ID of the room.
   - `stream_ordering` - integer -  The stream ordering of the most recent
-    successfully-sent [PDU](understanding_synapse_through_grafana_graphs.md#federation)
+    successfully-sent [PDU](../understanding_synapse_through_grafana_graphs.md#federation)
     to this destination in this room.
 - `next_token`: string representing a positive integer - Indication for pagination. See above.
 - `total` - integer - Total number of destinations.

docs/usage/administration/admin_api/registration_tokens.md

@@ -6,7 +6,7 @@ registration requests, as proposed in
 and stabilised in version 1.2 of the Matrix specification.
 To use it, you will need to enable the `registration_requires_token` config
 option, and authenticate by providing an `access_token` for a server admin:
-see [Admin API](../admin_api).
+see [Admin API](../admin_api/).
 
 
 ## Registration token objects