docs(aio): add angularfirebase.com to education resources

[codediodeio]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ X] The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• [N/A] Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

AngularFirebase.com added to the educational resources list.

As a resource for developers, it includes:

• Fast paced high-quality content for intermediate-level developers.
• YouTube channel with 7K subscribers and weekly updates.
• Slack team with 600+ members
• Enjoyed by Angular core members https://twitter.com/_davideast/status/928341941527261184
• And a bunch of other stuff

Does this PR introduce a breaking change?

[ ] Yes
[X] No

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