docs: add design doc for rule based index selection

[xuyifangreeneyes]

What problem does this PR solve?

Issue Number: close #27227

Release note

None

[ti-chi-bot]

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• qw4990
• winoros

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[tisonkun]

/cc @winoros

[tisonkun]

/cc @bb7133 @yudongusa

I think this proposal may require a release note as user should be aware that we improve performance in several scenarios notably.

[winoros]

@tisonkun Release note has contained this one.

[tisonkun]

/uncc @bb7133 @yudongusa

/cc @rebelice @qw4990

Please take a look at the late design document. Thanks!

[tisonkun]

@winoros @qw4990 could you please help on merging this PR?

[winoros]

/merge

[ti-chi-bot]

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

Commit hash: d69699d

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