docs: add firefox arm64 build factory

[MikeMcC399]

• follows on from 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

The above is not yet reflected in the factory/README documentation.

Change

Add information to the following, to cover the added support and availability of Firefox for Linux/arm64 in release 136.0 and above, and to explicitly list other individual browsers availability in Linux/arm64 images:

• factory/README
• factory/.env

Note that updates to other README documents are delayed until regular Cypress Docker images cypress/browsers and cypress/included publication is triggered by any new browser release, a new Node.js LTS release or a new Cypress release. This is likely to happen in the next week or two. Edit: see PR #1315

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[jennifer-shehane]

Thanks!