docs: Remove direct placement option

[morgo]

What problem does this PR solve?

Issue Number: ref #18030

Problem Summary:

This updates the proposal for Placement Rules in SQL to remove direct placement, which was requested by the product mangement team. When removing direct placement, there are other changes that need to be made for the design to make sense (i.e. infoschema.placement_rules doesn't make sense. So it is renamed to infoschema.placement_policies).

This also updates the design doc for #31093 by @lcwangchao

Sub-tasks required to adjust the design will be added to #18030

What is changed and how it works?

Direct Placement options are no longer supported by Placement rules in SQL. Please use a placement policy instead.

Check List

Tests

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

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility (experimental feature)

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

Direct Placement options are no longer supported by Placement rules in SQL. Please use a placement policy instead.

[ti-chi-bot]

[REVIEW NOTIFICATION]

This pull request has been approved by:

• mjonss
• xhebox

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/ef45800bf2cd2fb2ec6bc28b6f4c93a5b506e546

[mjonss]

LGTM

Just some minor nitpicking (including fixing older issues).

[xhebox]

I have no other concerns for now, but let us wait for eason.

[easonn7]

LGTM

[morgo]

/merge

[ti-chi-bot]

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

Commit hash: 114b20e

[ti-chi-bot]

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