doc: embed discourse titles as link text (stable-5.0) (#14317)

- Cherry-picked update to doc/installing.md from #14295
- Updated doc/reference/remote_image_servers manually per different
links set from main branch
- The three other Discourse links in main branch not backported per the
pages do not exist in stable-5.0

doc/installing.md

@@ -1,5 +1,5 @@
 ---
-discourse: ubuntu:37214, ubuntu:37327
+discourse: ubuntu:[Managing the LXD snap package](37214), ubuntu:[Building custom LXD binaries for side loading into an existing snap installation](37327)
 ---
 
 # How to install LXD

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
 ---