doc(source): doc for data source

[tabVersion]

Signed-off-by: tabVersion tabvision@bupt.icu

What's changed and what's your intention?

PLEASE DO NOT LEAVE THIS EMPTY !!!

Please explain IN DETAIL what the changes are in this PR and why they are needed:

as title, add data source design doc

• Summarize your change (mandatory)
• How does this PR work? Need a brief introduction for the changed logic (optional)
• Describe clearly one logical change and avoid lazy messages (optional)
• Describe any limitations of the current code (optional)
  • diagram in SVG format will be available soon

Checklist

• I have written necessary docs and comments
• I have added necessary unit tests and integration tests

Refer to a related PR or issue link (optional)

[codecov]

Codecov Report

Merging #1781 (1eb9035) into main (af40194) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1781      +/-   ##
==========================================
- Coverage   71.35%   71.35%   -0.01%     
==========================================
  Files         605      605              
  Lines       78689    78689              
==========================================
- Hits        56150    56148       -2     
- Misses      22539    22541       +2     

Flag	Coverage Δ
rust	71.35% <ø> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/connector/src/base.rs	0.00% <ø> (ø)
src/connector/src/filesystem/file_common.rs	80.00% <0.00%> (-0.45%)	⬇️
.../src/executor/managed_state/aggregation/extreme.rs	89.89% <0.00%> (-0.27%)	⬇️
src/common/src/types/ordered_float.rs	23.70% <0.00%> (+0.19%)	⬆️

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