docs: use sentence case for headers

[tomarra]

Status

READY

Description

Per the marketing content guidelines (see #576), headers should use sentence case, not Title Case. Fixes 169 headings across 44 files.

This one took more digging than the others — Vale's capitalization rule has some real quirks I only found by testing directly against it rather than guessing. Details below since they matter for review.

Left untouched — compound product/brand names where fixing the flagged word in isolation makes the phrase read worse than leaving it alone (e.g. "Azure key Vault", "app Store connect"):

• "Azure Key Vault", "GCP Cloud KMS", "Create a release in App Store Connect"
• "How does this relate to Firebase Remote Config or Launch Darkly?"
• "How does this relate to Flutter Hot Reload?" (Flutter's own name for the feature)

Left untouched — three "Flutter vs. X" headings: Vale's capitalization rule flags any heading containing "vs" or "vs." regardless of what surrounds it (verified with isolated test cases — even a heading of just "Testing vs here" gets flagged). This isn't configurable via the exceptions list; it's baked into the rule. The content itself (Flutter vs. the competition, Flutter vs. native development) is already correctly cased; Flutter vs. React Native is left as-is since "React Native" is itself a proper noun.

Left untouched — four "code push" FAQ headings (e.g. "Does code push require the internet to work?"): enabling the Vocab-driven proper-noun rule (for the Shorebird/Flutter/Code Push guideline) has a side effect — it makes Vale's capitalization rule also flag any heading containing a mis-cased vocab term, even when the heading has no actual sentence-case violation. These four headings get fixed by #579 (capitalizing "code push" → "Code Push"), which resolves this side effect too. No separate edit needed here.

Reverted to Title Case — a 3-item numbered list ("1. The Status Enum", "2. The Single State Class", "3. The Events"): oddly, Vale stops flagging #2 once lowercased but keeps flagging #1 and #3 despite identical structure (confirmed this is a real Vale inconsistency, not a mistake on my end). Rather than leave the list looking inconsistent, I reverted all three to their original Title Case.

New proper nouns discovered and added to the exceptions list (not previously recognized): Stripe, Fastfile, BuildContext, RenderObject, Apple, 1Password, AOT, GuardSquare, LTS, ExportOptions.plist, Skia, Impeller. Also fixed two headings using lowercase "fastlane" where the tool name should be capitalized ("Using Fastlane on CI/locally").

Follow-up needed: those new exceptions need to land in .vale/styles/Shorebird/Headings.yml on the tooling PR (#576) too — otherwise they'll show up as false positives again once both PRs merge. I'll push that update to #576 separately.

This is one of several small content PRs split out of #576 so the Shorebird.Headings check can eventually be made blocking without a giant, hard-to-review PR.

Test plan

• npm run build succeeds, no broken links
• cspell passes on all touched files
• Verified locally with the (not-yet-merged) Vale config from docs: add style checks for headings and content guidelines #576 (plus the new exceptions above) — only the 15 intentionally-left findings above remain