docs: add developer manual docs

[TomShawn]

docs: add developer manual docs

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[xtangcode]

Overall looks good to me. A few nitpicks:

• Consider extend the use-pl-container.md guide to to broader guide for "stored procedures & user-defined functions" which introduces stored procedures, user-defined functions supported in different languages, triggers, etc.
• Consider adding a data / tool integration section in the future.

[tuhaihe]

Thanks for your @TomShawn great work!

Left some comments for your information.