botserver: Document loading custom bots from Python files.

[ahmedforge]

Documents loading a custom Python bot by file path with --bot-name,
including configuration, relative and absolute paths, and troubleshooting.

This is a draft while awaiting feedback on the scope proposed in issue #636.
Related to #636.

How did you test this PR?

• Existing bot-loading and configuration tests: 2 passed.
• Configuration selection and relative/absolute file loading checked locally.
• git diff --check passed.
• Live message delivery through a custom bot has not been tested.

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.

[ahmedforge]

The Debian 11 / Zulip 4.0 compatibility check failed during provisioning:
apt could not download several Debian security packages (404 Not Found).
The other 13 checks passed.

This appears unrelated to the README-only change. Could a maintainer
advise whether this check needs a rerun or an environment update?