docs: restructure compute docs and add jobs documentation

[andre-code]

Summary

• Reorganizes user docs from Sessions into a concept-first Launchers & Compute section
• Adds new Job documentation (launcher, submit, monitor/cancel/dismiss jobs)
• Groups existing guides under Environment, Session, Job and Apps
• Updates content and links across docs to reflect the new UI with job launchers

New documentation structure

Launchers & Compute/
├── Launcher
├── Resource Pools & Classes
│
├── Environment/
│   ├── Environment (concept)
│   └── How to...
│       ├── Create environment with custom packages installed
│       ├── Create environment from private code repository
│       ├── Use your own docker image
│       └── Configure front ends for a session environment
│
├── Sessions/
│   ├── Session (concept)
│   └── How to...
│       ├── Add a session launcher
│       ├── CSCS integration
│       ├── Install packages on-the-fly
│       ├── Connect from local VSCode
│       └── Share a session launch link
│
├── Jobs/
│   ├── Job (concept)
│   └── How to...
│       ├── Add a job launcher
│       ├── Submit a job
│       ├── Monitor, cancel, and dismiss jobs
│       └── Handle long training runs
│
├── Apps/
│   └── How to...
│       ├── Set up Shiny app launcher
│       ├── Custom environment + launcher for Shiny
│       └── Create Streamlit app
│
├── Alerts/ (collapsed in sidebar)
│   ├── High Memory Usage
│   ├── Session Restarted (OOM)
│   └── Low Disk Space
│
└── How to... (cross-cutting)
    ├── Select compute resources
    └── Configure a session secret

[RenkuBot]

You can access the deployment of this PR at https://ci-renku-4518.dev.renku.ch

[eikek]

LGTM! 🎉

The base branch was changed.

[lokijuhy]

This is coming together very nicely! The docs for jobs themselves are spot on- I don't think I had any comments there. Just a few notes on adjustments for all the shuffling around of existing docs. Thank you!

[rokroskar]

I rebased on current master branch and made some wording edits.

[rokroskar]

Thanks all! I think the new structure looks really good!