Document repository name limits and restrictions

[webdevpraveen]

Resolves #44518

Description

This PR adds the missing documentation for repository name limits to the "Repository limits" page (content/repositories/creating-and-managing-repositories/repository-limits.md).

As outlined in the issue, the new Repository name section now explicitly documents:

• Maximum length: The 100-character limit.
• Allowed characters: Which characters are allowed and how non-ASCII characters are automatically sanitized.
• Reserved suffixes: The restrictions regarding .git and .wiki suffixes.

These limits are fundamental when creating a repository, so I've placed the new section right before the Repository size section for logical flow.

[github-actions]

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Source	Review	Production	What Changed
repositories/creating-and-managing-repositories/repository-limits.md	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17	fpt ghec ghes@ 3.21 3.20 3.19 3.18 3.17

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