[dmt] feat: add no-lang-key rule for documentation front matter

[fuldaxxx]

Description

Add a new no-lang-key rule to the documentation linter that checks markdown files in docs/ for the presence of a lang: key in YAML front matter. The rule parses the front matter block (between --- delimiters) and reports an error with the exact line number if lang: is found.

Why do we need it, and what problem does it solve?

The language of a documentation file is already determined by the file naming convention: .md for English and .ru.md for Russian. Having an additional lang: key in the YAML front matter is redundant and can lead to inconsistencies -- for example, a file named README.md (English) could have lang: ru in its front matter, or vice versa. This creates confusion for the documentation build system and for contributors.
This rule enforces a single source of truth for the document language (the file name suffix) and ensures that the lang field is removed from front matter across all module documentation.

Example

🐒 [no-lang-key (#documentation)]
     Message:      Documentation contains 'lang' key in front matter; this field should be removed
     Module:       test-module-with-lang-key
     Value:        Line 3: front matter contains 'lang' key which should be removed
     FilePath:     docs/CONFIGURATION.ru.md
🐒 [no-lang-key (#documentation)]
     Message:      Documentation contains 'lang' key in front matter; this field should be removed
     Module:       test-module-with-lang-key
     Value:        Line 3: front matter contains 'lang' key which should be removed
     FilePath:     docs/README.ru.md