[#242] No scan symlinks as md files

[aeqz]

Problem: when the repository contains a symlink to a markdown file, it is processed by xrefcheck as if it was the same markdown file but in the symlink's location. This leads to broken references and can be avoided because neither GitHub nor GitLab try to render symlinks as the file they point to.

Solution: consider symlinks as no scannable files. In the future, we will consider to include a new dedicated scanner for symlinks if it works.

Related issue(s)

Fixes #242 Relates #244

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Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
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  • Haddock

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  • [x] Any modifications of public contracts comply with the Evolution of Public Contracts policy.
  • [x] I added an entry to the changelog if my changes are visible to the users and
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• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.

[aeqz]

After several attempts on where to ignore symlinks, I have implemented the simplest one and we will complicate it in a following PR for #244 if necessary.