Expanded setup instructions

[rober-m]

Expanded setup instructions related to Nix and Docker, and added the option to use Demeter.

The new information is based on the Education team's plinth-dev-env development environment, which was previously used during the PPP and other training courses and events.

The objective of moving those instructions here is, in part, to be able to finally fully drop plinth-dev-env in favor of plinth-template as the "development environment of choice" for Plinth.

Pre-submit checklist:

• Branch
  • Tests are provided (if possible)
  • Commit sequence broadly makes sense
  • Key commits have useful messages
  • Changelog fragments have been written (if appropriate)
  • Relevant tickets are mentioned in commit messages
  • Formatting, PNG optimization, etc. are updated
• PR
  • (For external contributions) Corresponding issue exists and is linked in the description
  • Targeting master unless this is a cherry-pick backport
  • Self-reviewed the diff
  • Useful pull request description
  • Reviewer requested

[rober-m]

Hi @Unisay, I'd appreciate a review whenever you have the time. 🙏

[rober-m]

Hi, @LukaKurnjek, as the original author of the instructions, feel free to take a look if you have the time. I tried to be as concise as possible and offloaded much of the original notes and instructions to their respective docs. I hope that's ok.

[Copilot]

Pull Request Overview

This PR expands the setup instructions in the README to provide more detailed and organized guidance for setting up development environments. The changes restructure existing content into collapsible sections and add a new Demeter cloud development option.

• Reorganized existing Nix and Docker setup instructions into collapsible details sections for better readability
• Added comprehensive Demeter cloud development environment setup instructions
• Enhanced Docker/Devcontainer instructions with more specific guidance for different use cases

[LukaKurnjek]

Looks OK to me. Feel free to merge it.

[Unisay]

LGTM!

@zeme-wana waiting for you to take a look too :-)