CHANGELOG: use reference style links (#325)

Originally we stopped doing links in the CHANGELOG since it harmed
readability when reading the raw markdown file. However we can use
reference styles links instead.

* Pros
  - Doesn't harm readability.
  - We can link to proper thread on the proper repo (`CodeBerg/nsxiv`
    for newer threads, `nsxiv-record` for wiped ones)
  - The links will be clickable on the [webpage]
  - Both GitHub and Codeberg support reference style links, as well as
    lowdown (which we use for building the webpage)

[webpage]: https://nsxiv.codeberg.page/changelog/

* Cons
  - Slightly higher maintainance burden.
  - Larger file size.

Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/325
Reviewed-by: Berke Kocaoğlu <berke.kocaoglu@metu.edu.tr>

CONTRIBUTING.md

@@ -74,7 +74,8 @@ For releases, the process is the following:
       changes which never made it into a release.
     * Use the "Changes" section to document behavior changes since the last
       release, the "Added" section for new features, and the "Fixes" section
-      for fixed bugs or regressions. Include pull request IDs.
+      for fixed bugs or regressions.
+    * Include pull request IDs with reference style links.
 
 Mirroring to GitHub: