Docs: Plugin E2E - Update docs

[sunker]

What this PR does / why we need it:

Now that create-plugin will scaffold Playwright and grafana/plugin-e2e instead of Cypress based grafana/e2e, the docs needs to be updated accordingly. This PR does the following changes:

• there's a new Migrating from grafana/e2e article. This guide walks through how to manually install Playwright and plugin-e2e, how to migrate tests, setup CI and how to uninstall grafana/e2e.
• the Get started guide is now focused on plugins that have been scaffolded with plugin-e2e setup. only walks through the steps needed to run the specs that were generated by create plugin.
• removing all notices about plugin-e2e being in beta
• adding notice about grafana/e2e being deprecated

Do not merge before this PR is merged.

Which issue(s) this PR fixes:

Part of https://github.com/grafana/grafana-plugins-platform-team/issues/105

Fixes #

Special notes for your reviewer:

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[sympatheticmoose]

Added some comments, but overall lgtm. I've not tested links or the actual steps (but don't think theres many new instructions if any as part of this)

[sympatheticmoose]

We could warn here about issues targeting versions greater than 11 using cypress

[sunker]

I don't think it's necessary. We've already made it clear at the top of the page that the package won't be supported after G11.

[sympatheticmoose]

I think theres a difference between not being supported and not working entirely, people might just expect that no new features would be added and fixes not applied. But not a blocker by any sense

[jackw]

Nice work @sunker ! 👏

Nothing blocking here IMHO but caught a few typos.

[sunker]

Changes applied. Thanks for feedback everyone!