docs(code-push): add runtime error reference page

[eseidel]

What

Adds a new page to the Code Push docs: Error Reference (code-push/runtime-errors.mdx), explaining the runtime errors and exceptions most commonly reported by apps that use the optional shorebird_code_push package.

This is the first reference-style page in the docs and the first page focused on the package's runtime behavior (the API itself remains documented on pub.dev).

Framing

Written as operational documentation, not an API reference:

• The shorebird_code_push package is called out as optional (Shorebird patches automatically without it).
• Notes that Shorebird has no built-in crash/error reporting — you only see these errors if you've wired up your own (links to the existing Sentry/Crashlytics pages).
• Full API signatures are deferred to pub.dev; this page only helps interpret errors.
• Lives entirely under code-push/ (no new top-level section).

Contents

• How to read an UpdateException (<message> / <reason>), with a table of UpdateFailureReason values.
• Errors grouped by class: benign (noUpdate, Update already in progress), network/transient, device/environment, and patch-state.
• noUpdate guidance: it's obsolete — upgrade shorebird_code_push to 2.0.6+.
• Cross-linked from the Troubleshooting page.

Verification

npm run build (astro check + build) passes with 0 errors / 0 warnings, and the internal-link validator reports all links valid.

[eseidel]

Closing in favor of fixing the root cause: the four-value UpdateFailureReason enum collapses most real failures (network, permissions, disk-full, server) into unknown, and this doc only existed to help humans do the classification the enum should do. We're going to plumb richer, categorized failure reasons through the Rust updater and the shorebird_code_push Dart layer instead. May revisit a slimmed-down docs page afterward.