chore(deps): upgrade dependencies (DOCSDEV-69)

- astro 6.1.8 → 6.3.8 (pinned ~6.3.8; astro 6.4 incompatible with
  Starlight 0.39.2's pinned @astrojs/mdx ^5.x)
- @astrojs/starlight 0.38.3 → 0.39.2
- @astrojs/mdx 5.0.3 → 5.0.6
- @astrojs/node 10.0.5 → 10.1.2
- @interledger/docs-design-system 0.12.0 → 0.13.0
- starlight-links-validator 0.23 → 0.24
- html-to-text 9.0.5 → 10.0.0 (MAJOR — single call site in
  src/utils/create-excerpt.js; convert+selectors API is stable across
  9→10; build-time excerpt generation verified)
- markdown-it 14.1.1 → 14.2.0
- eslint 10.2.1 → 10.4.0, globals 17.5 → 17.6
- @typescript-eslint/parser, typescript-eslint 8.59 → 8.60

Author: Infi-Knight