Fix headers with custom IDs via the correct syntax (#1578)

Was previously using <a name="..."> elements which just
add another anchor rather than changing the existing one.
They also use a deprecated HTML attribute and in some cases
broke the header display.

Fixes #1572.

Signed-off-by: Midnight Veil <midnightveil@fea.st>

changelogs/client_server/newsfragments/1578.clarification

@@ -0,0 +1 @@
+Fix broken header formatting. Contributed by @midnightveil.

changelogs/server_server/newsfragments/1578.clarification

@@ -0,0 +1 @@
+Fix broken header formatting. Contributed by @midnightveil.

content/client-server-api/modules/end_to_end_encryption.md

@@ -134,8 +134,7 @@ are offline/unable to upload additional keys, though sessions started using
 fallback keys could be vulnerable to replay attacks.
 {{% /boxes/warning %}}
 
-Devices will be informed, [via
+Devices will be informed, [via `/sync`](#e2e-extensions-to-sync), about the number of
-`/sync`](#a-namee2e-extensions-to-sync-extensions-to-sync), about the number of
 one-time keys remaining that can be claimed, as well as whether the fallback
 keys have been used. The device can thus ensure that, while it is online, there
 is a sufficient supply of one-time keys available, and that the fallback keys
@@ -1699,7 +1698,7 @@ When a client is updating a Megolm session in its store, the client MUST ensure:
 
 {{% http-api spec="client-server" api="keys" %}}
 
-##### <a name="e2e-extensions-to-sync"> Extensions to /sync
+##### Extensions to /sync {#e2e-extensions-to-sync}
 
 This module adds an optional `device_lists` property to the [`/sync`](/client-server-api/#get_matrixclientv3sync)  response,
 as specified below. The server need only populate this property for an

content/server-server-api.md

@@ -877,8 +877,7 @@ on the resulting `m.room.member` event.
 If the joining server fails all conditions then a 403 `M_FORBIDDEN` error
 is used by the resident server.
 
-<a name="knocking-rooms">
+## Knocking upon a room {#knocking-rooms}
-## Knocking upon a room
 
 Rooms can permit knocking through the join rules, and if permitted this
 gives users a way to request to join (be invited) to the room. Users who