feat: improve search relevance by indexing headings and removing location penalty

[karenjli]

Summary

Improves search by indexing page headings and removing the Fuse.js location-based scoring penalty.

Changes

• Extract headings from tableOfContents and add as a searchable headings field
• Set ignoreLocation: true so matches anywhere in a document are scored equally
• Add unit tests for the new flattenHeadings helper

Why

By default, Fuse.js penalizes matches far from position 0. For long body fields, this makes most content effectively unsearchable. Even exact matches deep in a page can get filtered out. Adding headings as a searchable field further improves discoverability of section-level content.

[wraithgar]

The test file isn't parsing in jest:

Details:

    /home/runner/work/documentation/documentation/src/hooks/use-search.js:182
        worker.current = new Worker(new URL('../util/search.worker.js', import.meta.url));
                                                                               ^^^^

    SyntaxError: Cannot use 'import.meta' outside a module

    > 1 | import {flattenHeadings} from '../use-search'