doc: embed discourse post title as link text (#14295)

This PR explicitly specifies the link text for Discourse posts (from the
posts' titles) instead of extracting it at build time. Recent Discourse
outages have caused build failures when the link text couldn't be
fetched dynamically. Hard-setting the link text should prevent this.

**Note:** Docs on specifying link text
[here](https://github.com/canonical/canonical-sphinx-extensions/?tab=readme-ov-file#specify-links-for-a-page).
The docs also describes a backup link text option, but I did not use it
due to buggy behavior. _My expectation:_ The backup link text will be
used if the link text cannot be extracted dynamically, and the build
will not fail. _Reality:_ The backup link text is recognized, yet the
build still fails.

doc/explanation/authorization.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:41516
+discourse: ubuntu:[Identity and Access Management for LXD](41516)
 ---
 
 (authorization)=

doc/howto/snap.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:37214
+discourse: ubuntu:[Managing the LXD snap package](37214)
 ---
 
 (howto-snap)=

doc/installing.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:37327
+discourse: ubuntu:[Building custom LXD binaries for side loading into an existing snap installation](37327)
 ---
 
 (installing)=

doc/reference/remote_image_servers.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:43824,16647
+discourse: ubuntu:[New LXD image server available (images.lxd.canonical.com)](43824),[Image server infrastructure](16647)
 relatedlinks: https://www.youtube.com/watch?v=pM0EgUqj2a0
 ---
 

doc/reference/uefi_variables.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:42313
+discourse: ubuntu:[LXD VM instance EFI Variables edit CLI](42313)
 ---
 
 # UEFI variables for VMs