[dmt] feat: add markdownlint documentation rule

[fuldaxxx]

Description

Adds a new markdownlint rule to the documentation linter. The rule checks .md files in a module's docs/ directory for compliance with a unified markdown style (Deckhouse conventions) using the go-markdownlint (https://github.com/ldmonster/go-markdownlint) library.

What the rule does:

• Recursively scans all .md files under docs/ (including nested ones, e.g. docs/internal/...)
• Runs the built-in markdownlint rules with a fixed set of deckhouse-specific overrides (line-length limits, blanks-around-headings, duplicate-heading siblings, consistent code-fence/emphasis/strong styles, trailing newline, etc.)
• Reports the rule name(s), description, file path and line number for each violation

Changes:

• pkg/linters/docs/rules/markdown.go — MarkdownRule implementation with deckhouseMarkdownlintConfig()
• pkg/linters/docs/documentation.go — invoke the new rule in Documentation.Run()
• pkg/config.go, pkg/config/global/global.go, internal/module/module.go — add MarkdownlintRule to the config and map the impact level
• go.mod/go.sum — github.com/ldmonster/go-markdownlint dependency
• pkg/linters/docs/README.md — rule documentation
• test/e2e/testdata/documentation/markdownlint-violations/ — e2e test case (duplicate h1 + missing trailing newline)

Example

🐒 [bilingual (#documentation)]
     Message:      Russian counterpart is missing: need to create a matching .ru.md in docs/
     Module:       test-markdownlint
     FilePath:     docs/README.md

🐒 [markdownlint (#documentation)]
     Message:        MD001/heading-increment Heading levels should only increment by one level at a time
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     8

🐒 [markdownlint (#documentation)]
     Message:        MD009/no-trailing-spaces Trailing spaces
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     3

🐒 [markdownlint (#documentation)]
     Message:        MD012/no-multiple-blanks Multiple consecutive blank lines
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     10

🐒 [markdownlint (#documentation)]
     Message:        MD018/no-missing-space-atx No space after hash on atx style heading
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     1

🐒 [markdownlint (#documentation)]
     Message:        MD022/blanks-around-headings Headings should be surrounded by blank lines
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     7

🐒 [markdownlint (#documentation)]
     Message:        MD022/blanks-around-headings Headings should be surrounded by blank lines
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     8

🐒 [markdownlint (#documentation)]
     Message:        MD041/first-line-heading/first-line-h1 First line in a file should be a top-level heading
     Module:         test-markdownlint
     FilePath:       /tmp/dmt-test-md/docs/README.md
     LineNumber:     1