Custom Agents Insights Module Instrumentation (#14133)

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## DESCRIBE YOUR PR
Documentation how to manually instrument your AI Agent so the data shows
up in the new "Agents Insights Module".

Preview:

https://sentry-docs-git-antonpirker-agents-module-custom-instrum-ba897a.sentry.dev/platforms/python/tracing/instrumentation/custom-instrumentation/ai-agents-module/

## IS YOUR CHANGE URGENT?  

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to go live.
- [x] Urgent deadline (GA date, etc.): 2025-06-26
- [ ] Other deadline: <!-- ENTER DATE HERE -->
- [ ] None: Not urgent, can wait up to 1 week+

## SLA

- Teamwork makes the dream work, so please add a reviewer to your PRs.
- Please give the docs team up to 1 week to review your PR unless you've
added an urgent due date to it.
Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [x] Checked Vercel preview for correctness, including links
- [x] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)

## LEGAL BOILERPLATE

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Look, I get it. The entity doing business as "Sentry" was incorporated
in the State of Delaware in 2015 as Functional Software, Inc. and is
gonna need some rights from me in order to utilize my contributions in
this here PR. So here's the deal: I retain all rights, title and
interest in and to my contributions, and by keeping this boilerplate
intact I confirm that Sentry can use, modify, copy, and redistribute my
contributions, under Sentry's choice of terms.

## EXTRA RESOURCES

- [Sentry Docs contributor guide](https://docs.sentry.io/contributing/)

Author: antonpirker