doc: updated developer doc readme

[hengm3467]

What's changed and what's your intention?

Add a brief intro and a reference to the user documentation.

PLEASE DO NOT LEAVE THIS EMPTY !!!

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

• 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)

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 #1715 (d60a8e0) into main (1da4127) will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1715      +/-   ##
==========================================
+ Coverage   71.15%   71.17%   +0.02%     
==========================================
  Files         598      598              
  Lines       77556    77534      -22     
==========================================
+ Hits        55182    55183       +1     
+ Misses      22374    22351      -23     

Flag	Coverage Δ
rust	71.17% <ø> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
src/bench/ss_bench/operations/write_batch.rs	0.00% <0.00%> (ø)
src/common/src/types/ordered_float.rs	23.70% <0.00%> (+0.19%)	⬆️
src/meta/src/hummock/mock_hummock_meta_client.rs	46.66% <0.00%> (+13.01%)	⬆️

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