Improve issue deduplication strategy for generate-releases

> Non-blocking: Are we concerned about titles being possibly renamed? I doubt that would happen very often (if at all), but it might be somewhat difficult if we ever want to change the title format to something else.
>
> This might be better served as a followup, but I've seen people use hidden comments at the start of the issue body to contain metadata about the issue. That might make any future migrations a bit easier.

_Originally posted by @MrGVSV in https://github.com/bevyengine/bevy-website/pull/1222#discussion_r1621291017_

Start by locating the generate-releases entry point and reading how it currently identifies duplicate issues. Define the desired behavior for renamed titles and hidden metadata, then document or verify a deduplication strategy that remains workable during future format changes.

Written by the indexing model from the issue text.