ci: split link checking into multiple jobs

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Description

Once pr-check-links is run, GitHub CI started returning Error: Compiled regex exceeds size limit of 10485760 bytes..

Apparently, the file .lycheeignore contains too many entries causing lychee to construct an oversized regex.

I tried several solutions:

• Tried using --exclude for links instead of .lycheeignore,however, the pipeline resulted in failure.
• Tried other countless fixes invain

The only working solution appeared to be splitting link checking into several stages:

• non-blog - all files in ./content except for ./content/blog/ and README.md
• blog_0-9a-f - all files in ./content/blog/[0-9a-f]*/ folders
• blog_g-z- all files in ./content/blog/[g-z]*/ folders

Related

• Issue Lechee check on PR is checking whole portal, instead of just modified content #101

Testing

Extensive testing was done in the testing environment.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[horw]

Maybe in the future, it makes sense to move to our own runner to avoid this limitation for the regex buffer on GitHub's side. I am not sure whether this issue will disappear or not, but we can try it in the future.

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Fixes #409