Refactor front page of the documentation

[ankur22]

This will be based on and following on from the work that was done in the following branches and PRs:

• Open Branches:

  • https://github.com/grafana/k6-docs/commits/xk6-browser-new-format
  • https://github.com/grafana/k6-docs/commits/docs/582-xk6-browser

• PRs:

  • xk6-browser API documentation k6-docs#642
  • WIP - xk6-browser updates k6-docs#508

1. Add some details of what Launch is and does, and how it ties in with BrowserType. We need to link this from the Browser page too.
2. Replace the MISSING PLAYWRIGHT APIS column with a Description column on the front page (Refactor the browser class docs for xk6-browser k6-docs#722 (comment))
3. Remove the state-ness of the classes.