docs: add mdx file linting, fixup all mdx-related errors

[mikehardy]

Description

There were some problems in the conversion from previous Gatsby static website build and dynamic docs.page mdx build - specifically some of the files had invalid MDX

This PR adds an MDX linter to our eslint config, and configures our lint:markdown run target to use it, then fixes all errors that turned up

An unfortunate discovery was that when I previously converted our eslint config file from the legacy format to the new "flat config" format I did so in a compatibility mode that was a real hash - I paid up the technical debt and forward-ported us to a really clean eslint "flat config" style in the first commit so that I could add the mdx file easily

Related issues

• @russellwheatley noticed that rnfirebase.io/messaging/usage had an error and that led to this rabbit hole
• Library consumer also noticed - Fixes [📚] Cloud Messaging document can't be opened #8987

Release Summary

docs only, will go live once merged to main

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
  • Yes
• My change supports the following platforms;
  • Android
  • iOS
  • Other (macOS, web)
• My change includes tests;
  • e2e tests added or updated in packages/\*\*/e2e
  • jest tests added or updated in packages/\*\*/__tests__
• I have updated TypeScript types that are affected by my change.
• This is a breaking change;
  • Yes
  • No

Test Plan

1- hit rnfirebase.io/messaging/usage and it should work (can check the preview link as well, and should work --> https://rnfirebase.io/~8993/messaging/usage )
2- yarn lint:markdown should also pass, and will be exercised in CI (or when tested locally) to make sure nothing creeps in over time without us knowing

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Project	Deployment	Actions	Updated (UTC)
react-native-firebase	Error		Apr 23, 2026 0:30am

[gemini-code-assist]

Code Review

This pull request adds an 'AI Logic' group to the documentation, updates versioning language in MDX files, and migrates the ESLint configuration to the Flat Config format. Feedback focuses on correcting the ESLint configuration, where the extends key is invalid and configs.all was unintentionally enabled. Additionally, logical errors were identified in the Android and iOS installation guides regarding manual integration requirements, and the markdown linting glob was found to be too restrictive, potentially excluding standard .md files.