docs: add transaction savepoint design doc (#36457)

[ti-srebot]

cherry-pick #36457 to release-6.2
You can switch your code base to this Pull Request by using git-extras:

# In tidb repo:
git pr https://github.com/pingcap/tidb/pull/36728

After apply modifications, you can push your change to this PR via:

git push git@github.com:ti-srebot/tidb.git pr/36728:release-6.2-324b5ee0c7ec

---

What problem does this PR solve?

Issue Number: close #36456

related google doc: https://docs.google.com/document/d/1sncyK1dNqBvI0aH_lAgnPnDa5c8bSbenUFE2v-g7ouo/edit?usp=sharing

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Documentation

related doc: pingcap/docs#9593

Release note

Please refer to Release Notes Language Style Guide to write a quality release note.

None

[ti-chi-bot]

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[ti-chi-bot]

@ti-srebot: This cherry pick PR is for a release branch and has not yet been approved by release team.
Adding the do-not-merge/cherry-pick-not-approved label.

To merge this cherry pick, it must first be approved by the collaborators.

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[ti-srebot]

/run-all-tests

[ti-srebot]

@crazycs520 you're already a collaborator in bot's repo.

[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/afec4ab9b92dde163cfcd292c1f51a47c1e3d700

[xhebox]

closed for DMR version