test(docs): add documentation on custom Ionic functions (#27248)

Issue number: N/A

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This PR adds documentation for the following:

1. A step-by-step guide for migrating to generators.
2. Documentation on the custom tooling we built on top of Playwright
3. A best practice guide. Note: This was originally in the Ionic
internal Notion, but I moved it here because it is of value to community
contributors.

I also added a few new best practices that were not on the Notion guide:

- Components must have a `basic` test with an `index.html` file
- Slow tests should be broken up across multiple files for CI
performance
- Rendering and functionality tests should be broken up across multiple
test.describe blocks or files

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Co-authored-by: Amanda Johnston <90629384+amandaejohnston@users.noreply.github.com>