Configure markdownlint to allow 2-space hard line breaks

[Copilot]

Description

Threat model files in System.Private.Uri/docs use 2 trailing spaces for hard line breaks (standard markdown syntax). Configured MD009 rule to recognize this instead of suppressing entire files.

Changes

• Set br_spaces: 2 in .markdownlint.json to allow 2-space line breaks
• Removed file-based suppression approach (.github/workflows/markdownlintignore)
• Reverted workflow changes that referenced the removed ignore file

This properly handles the semantic meaning of trailing spaces rather than blanket-suppressing files from linting.

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[MihaZupan]

@copilot Let's configure markdownlint to ignore 2 spaces instead of suppressing individual files
See https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md#md009---trailing-spaces
The two spaces in this case are hard line breaks, not random spaces

[Copilot]

@copilot Let's configure markdownlint to ignore 2 spaces instead of suppressing individual files
See https://github.com/markdownlint/markdownlint/blob/main/docs/RULES.md#md009---trailing-spaces
The ...

Done! Changed approach to configure MD009's br_spaces parameter to allow 2 trailing spaces (hard line breaks) instead of suppressing individual files. Removed the markdownlintignore file and reverted workflow changes. (5661256)