docs: introduce Firefox arm64 build

[MikeMcC399]

Follows on from:

• Add Firefox to Linux/arm64 browser images #1190
• feat: enable firefox arm64 build with cypress/factory #1307

Situation

• feat: enable firefox arm64 build with cypress/factory #1307 added:

  • the ability for cypress/factory to generate Docker images including Firefox arm64

• chore: update Firefox 136.0.2 Edge 134.0.3124.68-1 #1318 added:

  • Firefox 136.0.2 published as part of Linux/arm64 images

The above is not yet reflected in README documentation.

Change

Add information to READMEs to cover the added support and availability of Firefox for Linux/arm64 in release 136.0 and above.

[cypress-app-bot]

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