Restructure README: replace nested collapsible sections with flat headings and TOC

[dadigu]

Summary

• Replaces deeply nested <details>/<summary> sections with standard markdown headings
• Adds a table of contents for quick navigation
• Preserves all original copy, links, and code examples

Addresses #1061

Motivation

The current README nests content up to 3 levels deep in collapsible sections, which makes it difficult to:

• Know what content exists without expanding every toggle
• Link directly to specific sections

New users may miss important sections like "Root privileges" or "Known Issues" entirely because they're hidden behind nested toggles.

A flat heading structure with a TOC provides the same "jump to what you need" navigation without hiding content by default. Readers can still scan quickly via the TOC, and GitHub's built-in heading anchors make deep linking work naturally.

Changes

• Removed all <details>/<summary> wrappers
• Converted content to standard ## / ### / #### markdown headings
• Added a table of contents at the top with anchor links
• No copy, links, or code examples were changed

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[rgoldberg]

Thanks for the PR.

How's this as a compromise solution:

Keep all top-level accordions expanded by default, but all lower-level accordions collapsed by default.

Provide pairs of buttons to the right (or left) of each accordion:

1. expands/collapses all direct children of that accordion
2. expands/collapses all descendants of that accordion

One additional expand/collapse button pair would be above (and possibly one pair would be below) the top-level accordions to expand/collapse all accordions.

It would probably be useful to indent the titles (summaries) of each accordion according to its hierarchical depth enough so that its depth would be readily apparent.

Would you be able & willing to implement that?