Add basic support for in-app "Quick Start" guides

[abey79]

What

This PR lays the basis for in-app "Quick Start" guides.

After a failed attempt to use re_sdk from re_viewer (can't target wasm), this PR builds on recent StoreDb API improvements to create "quick start" recordings from scratch.

This involves:

• adding a nice StoreDb::from_info_and_rows() helper function
• adding SystemCommand::LoadStoreDb(StoreDb)
• adding StoreSource::Viewer (for recordings generated from the viewer itself)
• some helper functions in welcome_page.rs

This PR also introduces a draft for the (native-only) Python and Rust Quick Starts, and placeholder for the other languages.

To address (in follow-up PR):

• markdown formatting issue where H1 appears smaller than H2 (current work around: skip on using H1)
  • Markdown heading levels all have the same font size #3612
• specialise Quick Start for non-server Viewers
  • Quick start: add alternative content for non-serving viewers #3871
• write Quick Start for C++:
  • Add C++ to in-viewer quick-start guide #3870
• markdown formatting issue:
  • Title to body vertical space missing when an image precedes the title lampsitter/egui_commonmark#19

• Fixes Improve Welcome Screen's Getting Started section #3095

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[emilk]

I changed it to a draft because of the wasm-issue discussed in https://rerunio.slack.com/archives/C041NHU952S/p1697037259411659

[emilk]

Looks good, but let's pull out the actual example code to a separate file so that we make sure it will always run properly!