Add Dependabot permissions warnings

[Marcono1234]

Why:

Dependabot intentionally has no built-in automerge feature (dependabot/dependabot-core#1973 (comment)), and in the past permissions for Dependabot workflows were changed to read-only by default (changelog entry).

If I understand it correctly, the concern is that a Dependabot workflow with write permissions could be exploited by a compromised dependency to immediately compromise the consuming repository as soon as the Dependabot PR is created, without any interaction of the owner.

Therefore adding a custom automerge workflow for Dependabot or giving its workflows write permissions can be a security risk, and is probably worth pointing out in the documentation.

Slightly related to #37657, but does not resolve it

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

Add warnings to the documentation to inform users about the risk of giving Dependabot workflows more permissions

Check off the following:

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

Source	Review	Production	What Changed
code-security/dependabot/troubleshooting-dependabot/troubleshooting-dependabot-on-github-actions.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12
code-security/dependabot/working-with-dependabot/automating-dependabot-with-github-actions.md	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12	fpt ghec ghes@ 3.16 3.15 3.14 3.13 3.12

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

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[Marcono1234]

Please let me know if I am misunderstanding the security risk here, or if you would like the warnings to be changed.

[Sharra-writes]

@Marcono1234 Thanks for opening these! I'll get this triaged and look for an SME to review the specifics. We appreciate the time you're investing to improve the documentation. 💛

[github-actions]

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