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>

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