Improve general guidance aka. easy user journey

## About

> The developer documentation has grown organically over a decade. Its corpus is large, and for a first-time user of CrateDB, it can be difficult to find the information needed.

Improve relevant details.

## Details

- Overhaul "Getting Started" section.
- Provide tiny runnable examples to make pages actionable for newcomers.
- Rearrange the pages into a new menu better suited to support the user journey in adopting CrateDB. A few proposals for outlines exist already.
- Take the most critical pages supporting the user journey and clean them up. We also need to test the statements/snippets are correct.

## Background

- People have been voting for an easier user journey.
https://github.com/crate/tech-content/issues/143#issuecomment-3167655658

> Our goal is to produce clear, up-to-date, and working documentation of the data sources we support. Let’s keep it simple and focused, no need to explain basics (or insert low-value screenshots), as we’re writing for technically proficient users.

## References

- https://github.com/crate/tech-writing/issues/349
- https://github.com/crate/cratedb-guide/issues/10
- https://github.com/crate/roadmap/issues/32
- https://github.com/crate/cratedb-guide/issues/48
- https://github.com/crate/crate-tutorials/issues/61
- https://github.com/crate/tech-content/issues/42
- https://github.com/crate/cratedb-guide/issues/28
- https://github.com/crate/crash/issues/458

Start with the existing "Getting Started" section and review the outline proposals and linked documentation issues for the intended user journey. Identify the critical pages, create tiny runnable examples, and verify their statements and snippets; done means the navigation and guidance are reorganized, focused, current, and tested.

Written by the indexing model from the issue text.