Add blog introducing-gardenadm-bootstrap-for-autonomous-shoots

[gardener-answering-machine]

Purpose:
This is an automatically generated draft pull request proposing a new blog post based on a Gardener review meeting:

"Introducing gardenadm bootstrap for Autonomous Shoots"

The purpose is to inform the community about new Gardener features or changes, as discussed during review meetings.

Notes to Reviewers:
This draft was automatically generated by LLMs using the review meeting recording and referenced materials.
Please evaluate whether this topic is suitable for a blog post. If so, review and edit the content as needed.
If you decide the topic isn’t appropriate for a blog post, feel free to close this PR and delete the branch.

⚠️ This is an experimental GenAI feature. Feedback is welcome! Please direct it to @vlerenc. Thank you!

Instructions for Reviewers:

1. Clone the repository and change to the directory:

git clone https://github.com/gardener/documentation
cd documentation

2. Check out the branch:

git fetch origin && git checkout blog/2025-06-25-introducing-gardenadm-bootstrap-for-autonomous-shoots

3. Review the content in website/blog/2025/06-25-introducing-gardenadm-bootstrap-for-autonomous-shoots.md.
4. Make any necessary edits, additions, or removals and then push the changes:

git add website/blog/2025/06-25-introducing-gardenadm-bootstrap-for-autonomous-shoots.md
git commit --amend --no-edit
git push origin +blog/2025-06-25-introducing-gardenadm-bootstrap-for-autonomous-shoots

5. If the draft isn’t viable, close this PR and delete the branch.
6. If the post is ready for further review or publishing:
   • Mark this PR as Ready for review
   • Invite additional reviewers or merge the PR

Thank you for helping us share valuable updates from the Gardener project with the community!

[vlerenc]

Sorry @timebertt, I just noticed that you were not a collaborator for the documentation repo, so this proposed PR was hanging in the air. I added you now as collaborator, which also generally makes sense because of the steering meetings (cc @rfranzke @ScheererJ).