docs-builder: add pull-requests: write permission to docs-build workflow

[reakaleek]

Details

⚠️ This PR was created by an automated tool. Please review the changes carefully. ⚠️

Add permissions pull-requests: write to the docs-build workflow.

Why

We want to comment on the PR with the links to modified docs pages.

See elastic/docs-builder#1395 for further details

[botelastic]

This pull request doesn't have a Team:<team> label.

[github-actions]

🤖 GitHub comments

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Just comment with:

• run docs-build : Re-trigger the docs validation. (use unformatted text in the comment!)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @reakaleek? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.