Add known case with monorepos

[dsotirakis]

Why:

Closes: #38272
GitHub support ticket: #3380332

What's being changed (if available, include any code snippets, screenshots, or gifs):

Adds a small bit about update to monorepos where dependabot needs to take action. Looks like "namespaced" tags are better supported by git rather than tags with - as a separator.

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• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.

[github-actions]

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Pull Request Overview

This PR adds a documentation section for users of monorepos with GitHub Actions, explaining how tag formatting can impact Dependabot’s ability to properly detect and update action versions.

• Introduces a new "Known issue: Tag format for GitHub Actions in monorepos" section.
• Provides examples demonstrating why the dash (-) separator is not recommended and the slash (/) separator should be used.
• Offers a clear recommendation to users for proper tag format usage in monorepos.

[github-actions]

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Table of review links

Note: Please update the URL for your staging server or codespace.

The table shows the files in the content directory that were changed in this pull request. This helps you review your changes on a staging server. Changes to the data directory are not included in this table.

Source	Review	Production	What Changed
code-security/dependabot/ecosystems-supported-by-dependabot/supported-ecosystems-and-repositories.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13	fpt ghec ghes@ 3.16 3.15 3.14 3.13

Key: fpt: Free, Pro, Team; ghec: GitHub Enterprise Cloud; ghes: GitHub Enterprise Server

🤖 This comment is automatically generated.

[Sharra-writes]

Thanks for opening a PR! I'll get this triaged for review.

[github-actions]

Thanks for opening a pull request! We've triaged this issue for technical review by a subject matter expert 👀

[github-actions]

This is a gentle bump for the docs team that this PR is waiting for technical review.

[dsotirakis]

This is also now closed - #38272 (comment).
Could we please re-open it? (cc @Sharra-writes)