♻️ Refactor platform index page to include user-guide and runbook sections

[jasonBirchall]

👀 Purpose

• While working on platform documentation, I realised how awkward linking from the index page is. I believe this is partly because the index page does a fine job of describing the platform to someone who already knows what a platform is, but is terrible at guiding users through using our platform from scratch or even as a refresher.
• I hope the changes in this PR make linking more manageable and create a framework for good user engagement.
• I also hope that the team can start to publish some of its practices and ways of working.

♻️ What's changed

• The index page is heavily refactored and now includes two paths:
  -- A developer path: for a tenant using the platform, which comes under the guise of "user-guide"
  -- A platform engineer path: for the operators and engineers of the platform, intended for internal team use.

📝 Notes

• You can follow the instructions in the readme for a better look at the refactor, but I'll include screenshots below:

[AgaDufrat]

This sparks joy ✨ so neatly organised 🤩
I left a couple of minor comments.

[sihugh]

Looks like a good update, thanks 🚀

[sihugh]

Add an anchor link to the #contact-the-platform-engineering-team section? Or direct to the page? (And possibly in the next paragraph too)

[sihugh]

Not for now, but if this is more "platform" than Kubernetes, we probably want to remap the path to reflect that.