docs: move automerge deep dive to key concept section

[HonkingGoose]

Changes:

• Move existing automerge deep dive to key-concepts

Context:

Step one of making a key concepts page for the automerge feature.

I think the text needs some more work to make it beginner friendly.
Do you want me to make changes in this PR, or do we first want to move the document to its new place?

Helps with #11237, but does not close it.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please tick one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[rarkins]

Let's rename it to automerge.md

[HonkingGoose]

Let's rename it to automerge.md

Done! 😄

Do you want me to make some changes that I think are good right now, or do you first want to put the file in the correct place and fixup the sidebar on the publishing repository?

[HonkingGoose]

Do you want me to make some changes that I think are good right now, or do you first want to put the file in the correct place and fixup the sidebar on the publishing repository?

@rarkins I saw that you approved the PR, so I guess you're happy with the file as-is? The PR is still waiting to be merged though. Can you explain what you'd like me to do?

[renovate-release]

🎉 This PR is included in version 26.15.1 🎉

The release is available on:

• GitHub release
• 26.15.1

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