docs(size-analysis): Document Mobile Builds Monitors (#17485)

## DESCRIBE YOUR PR

Adds a new **Monitoring and Alerting** subsection to the Size Analysis
product page so users can discover [Mobile Builds
Monitors](/product/monitors-and-alerts/monitors/#mobile-builds-monitor-settings)
from the size analysis workflow.

The new section frames monitors as the way to wire up Slack (or other)
alerts on cadenced uploads — nightlies, weekly release branches — and
lightly contrasts them against PR-time Status Checks.

- Add `### Monitoring and Alerting` under `## Features`, immediately
after `### Status Checks`
- Deep-link to the Mobile Builds Monitor settings anchor in the Monitors
overview
- Two example workflows (nightly builds, weekly release branch)

Refs
[EME-1077](https://linear.app/getsentry/issue/EME-1077/document-mobile-builds-monitors-in-size-analysis-docs)

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## SLA

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Thanks in advance for your help!

## PRE-MERGE CHECKLIST

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

- [ ] Checked Vercel preview for correctness, including links
- [ ] 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)

Author: mtopo27

docs/product/size-analysis/index.mdx

@@ -82,6 +82,15 @@ Integrate into CI Size Analysis results on pull requests. You can track size on
 
 Learn how to set it up in the [CI integration guide](/product/size-analysis/integrating-into-ci/).
 
+### Monitoring and Alerting
+
+To get a Slack alert (or other notifications) when a build crosses a size threshold, use [Mobile Builds Monitors](/product/monitors-and-alerts/monitors/#mobile-builds-monitor-settings). Monitors complement [Status Checks](#status-checks) by alerting on cadenced uploads, not just PRs.
+
+Common workflows:
+
+- **Nightly builds** — alert when last night's build grew more than 2% vs. the prior night
+- **Weekly release branch** — alert when the release candidate exceeds an absolute size limit (e.g. 200 MB Download Size)
+
 ## Finding Uploaded Builds
 
 You can find uploaded builds through the Releases page. Builds will show up in two places: