feat: validate anchors in links

A markdown header may be attributed with [attr_list]. We use it for
linking to a specific header explicitly:

    ## Общие сведения {: #intro }
    См. [Общие сведения](#intro)

This patch enables links and headers validation:

- `check_headers.py` collects a list of existing anchors and checks
  there are no duplicates

- `check_links.py` validates links and checks the anchor exists

[attr_list]: https://python-markdown.github.io/extensions/attr_list/