integration-docs: Migrate docs closer to their source code.

[Niloth-p]

#documentation > README vs integration doc @ 💬

The integration docs of script integrations whose code is in this repository are moved from zulip/zulip to this repository to keep them closer to their integration scripts and READMEs.

Did not move the Trello doc even though there's a script in this repo, because it also has some webhook code in zulip/zulip, and it does not require downloading this repo package.

How did you test this PR?
Refer to the corresponding PR in zulip/zulip #34114.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Merged, thanks @Niloth-p!