doc: add foreign key design doc

[crazycs520]

Signed-off-by: crazycs520 [email protected]

What problem does this PR solve?

related issue: https://github.com/pingcap/tidb/issues/18209

Add foreign key design doc.

What is changed and how it works?

Check List

Release note

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Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/3f6439256445e08c9e58bd121588e37088c56c7c

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[crazycs520]

/merge

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This pull request has been accepted and is ready to merge.

Commit hash: b16233a90a44e9343e7311ecbfc62df7596e3dcd

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TiDB MergeCI notify

✅ Well Done! New fixed [1] after this pr merged.

CI Name	Result	Duration	Compare with Parent commit
idc-jenkins-ci/integration-cdc-test	🔴 failed 40, success 0, total 40	17 min	Existing failure
idc-jenkins-ci-tidb/mybatis-test	✅ all 1 tests passed	3 min 59 sec	Fixed
idc-jenkins-ci-tidb/integration-common-test	🟢 all 17 tests passed	11 min	Existing passed
idc-jenkins-ci-tidb/common-test	🟢 all 11 tests passed	10 min	Existing passed
idc-jenkins-ci-tidb/integration-compatibility-test	🟢 all 1 tests passed	7 min 33 sec	Existing passed
idc-jenkins-ci-tidb/tics-test	🟢 all 1 tests passed	6 min 12 sec	Existing passed
idc-jenkins-ci-tidb/integration-ddl-test	🟢 all 6 tests passed	5 min 12 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-2	🟢 all 28 tests passed	4 min 21 sec	Existing passed
idc-jenkins-ci-tidb/sqllogic-test-1	🟢 all 26 tests passed	4 min 11 sec	Existing passed
idc-jenkins-ci-tidb/plugin-test	🟢 build success, plugin test success	4min	Existing passed