Draft: Updated design and moved portal documentation

WIP

Technical changes were superseded by !565 (merged)

This MR is going to stay open until the design on resource requests is finalized

• Some design tweaks for latest HIFIS logos/colors
• Added devcontainer configs for local development
• Moved portal files into the integration directory as it's also there in the TOC
• Removed outdated documentation in the HCA/portal section
• Removed now outdated redirects

To do:

• Update architecture diagram
• Add portal docs linking wither to the readme in the repo or moving portal docs here
• Service integration guide for potential HCA users
  • Integration architecture overview
  • Resource lifecycle docs
  • How to write your own HCA plugin
  • What about AAI?