Follow-up for dbba4d1c

At very first start nothing works without entering the pipenv.
Now no need to remember it - you can simply run `make <target>`
and have fun.

Artur Sabirov authored 1 year ago and Yaroslav Dynnikov committed 1 year ago

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/