docs: clarify Dart symbols vs Shorebird engine symbols for crash reporting

[AbhishekDoshi26]

Adds a caution to Uploading Symbols explaining that Shorebird engine symbols (native libflutter.so / Flutter.framework frames) and Dart symbols (--obfuscate / --split-debug-info output) are separate uploads. Uploading only the --split-debug-info output with firebase crashlytics:symbols:upload leaves native engine crashes unsymbolicated. Also notes that Crashlytics needs Dart symbols uploaded before an obfuscated build reports a crash, so they should be uploaded as part of each build.

Adds an "Uploading engine symbols to Crashlytics" section covering Android (NDK crash reporting, unzipping each architecture's symbols.zip, firebase crashlytics:symbols:upload) and iOS (console dSYMs tab or the upload-symbols script).

Corrects the console section, based on the console source: the Artifacts tab only links the engine dSYM for iOS and macOS releases (not Android engine symbols) and needs the Developer role or higher (org or app collaborator), and its URL is now org-scoped (/orgs/<org-id>/apps/<app-id>/releases/<release-id>/artifacts; the old /apps/... URL returns 404). Notes that older Flutter versions name the iOS file Flutter.dSYM.zip.

Adds a short pointer to the same distinction near the top of the Crashlytics page.

Builds on #702 (Dart symbols for obfuscated builds).

[AbhishekDoshi26]

Self-review. One claim needs fixing and two wording points are worth tightening; details inline. Checked and fine: the #dart-symbols-for-obfuscated-builds anchor exists on main (#702), "the console or the URLs below" points at real content, the cspell:words comment matches the CI pages, and no agent entry points need updating since no command, flag, or workflow page is added.