docs: add developer manual docs

[TomShawn]

docs: add developer manual docs

---

Change logs

Describe your change clearly, including what problem is being solved or what document is being added or updated.

Contributor's checklist

Here are some reminders before you submit your pull request:

• Make sure that your Pull Request has a clear title and commit message. You can take the Git commit template as a reference.
• Learn the code contribution and doc contribution guides for better collaboration.
• Make sure that your changes deployment preview is successful.
• List your communications in the GitHub Issues or Discussions (if has or needed).
• Feel free to ask for the cloudberry committers or other people to help review and approve.

[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.

[xtangcode]

R is also a popular language among statisticians. Maybe helpful to add a guide for it. Not a high priority task at this moment though.