planner: add the design doc for the join order hint

[Reminiscent]

What problem does this PR solve?

Issue Number: ref #29932

Problem Summary:
add the design doc for the join order hint

What is changed and how it works?

add the design doc for the join order hint

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

Documentation

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

Release note

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

None

[ti-chi-bot]

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[sre-bot]

Code Coverage Details: https://codecov.io/github/pingcap/tidb/commit/50dcee14fdc65233732f21ac6c23c609cc3eed11

[codecov]

Codecov Report

Merging #34817 (50dcee1) into master (2106361) will increase coverage by 62.7876%.
Report is 4271 commits behind head on master.
The diff coverage is n/a.

Additional details and impacted files

@@               Coverage Diff               @@
##           master     #34817         +/-   ##
===============================================
+ Coverage        0   62.7876%   +62.7876%     
===============================================
  Files           0        850        +850     
  Lines           0     276553     +276553     
===============================================
+ Hits            0     173641     +173641     
- Misses          0      89129      +89129     
- Partials        0      13783      +13783