docs(rails): explain disabling agent, fixes #1466

[slhck]

What does this pull request do?

The option to enable the agent dynamically was asked for in #1466 – apparently present since 2020ish, but not documented everywhere, and not tested either.

Why is it important?

Some environemnts may not need apm testing (e.g., test). Users should discover this option more easily.

Checklist

• I have signed the [Contributor License Agreement](https://www.elastic.co/contributor-
  agreement/).
• My code follows the style guidelines of this project (See .rubocop.yml)
• I have rebased my changes on top of the latest main branch
• I have added tests that prove my fix is effective or that my feature works
• New and existing [unit tests](https://github.com/elastic/apm-agent-ruby/blob/
  main/CONTRIBUTING.md#testing) pass locally with my changes
• I have made corresponding changes to the documentation
• I have updated docs/release-notes/index.md – likely not worth adding there
• I have updated [docs/reference/supported-technologies.md](../docs/reference/
  supported-technologies.md) N/A
• Added an API method or config option? Document in which version this will be
  introduced N/A

Closes #1466

[smith]

Looks good. Will approve/merge once broken link is fixed.

[smith]

Thanks @slhck! I've got this merged. This will be included in an upcoming 4.10 release once we get some other outstanding PRs cleaned up.

[slhck]

Thank you!