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:

[github-actions[bot]]

Hello! 👋 This repository uses Auto for releasing packages using PR labels.

✨ This PR can be merged. It will not be considered when calculating future versions of the npm packages and will not appear in the changelogs.

[sunker]

Changes applied. Thanks for feedback everyone!