docs(documentation): switch heading style guidance to title case

- 810-documentation.mdc: in the Structure section, add a bullet
  that headings should use title case while preserving established
  acronyms and product names
- skills/documentation-standards/SKILL.md: replace the "Use sentence
  case" guidance under Headers with the same title-case + acronym/
  product-name rule, keeping the two surfaces in sync

Title case reads better in tables of contents, navigation rails,
and shared chats / emails where headings are excerpted; preserving
acronyms (API, JWT, OIDC) and product names (PostgreSQL, Kafka,
Cloudflare) avoids the visual noise of forcing them to title case.
The 810 rule and the documentation-standards skill are the two
canonical sources for documentation style; this aligns both.

Author: d-padmanabhan

rules/810-documentation.mdc

@@ -304,6 +304,7 @@ Prefer **neutral** or **imperative** phrasing over second-person voice.
 - Start with overview/context
 - Progress from simple to complex
 - Use headings to organize
+- Use title case for headings, preserving established acronyms and product names
 - Include table of contents for long docs
 
 ### Examples

skills/documentation-standards/SKILL.md

@@ -83,7 +83,7 @@ MIT - See LICENSE.
 ### Headers
 - Use `#` hierarchy (don't skip levels)
 - Keep headers concise
-- Use sentence case
+- Use title case for headings, preserving established acronyms and product names
 
 ### Code Blocks
 ````markdown