docs: Organize documentation and fix cargo doc warnings

Changes:
- Move development docs to docs/ directory:
  - IMPLEMENTATION_SUMMARY.md -> docs/IMPLEMENTATION_SUMMARY.md
  - MISSING_FEATURES.md -> docs/MISSING_FEATURES.md
  - features.md -> docs/features.md
- Move RELEASE_CHECKLIST.md to .github/ directory
- Remove RELEASE_NOTES.md (content is in CHANGELOG.md)
- Fix lifetime elision warning in src/writer/mod.rs:173
  - Add explicit lifetime parameter to TableBuilder return type

This resolves the documentation build failure in GitHub Actions
which was caused by the lifetime elision warning being treated as
an error due to RUSTDOCFLAGS="-D warnings".

Author: claude

RELEASE_CHECKLIST.md renamed to .github/RELEASE_CHECKLIST.md

@@ -170,7 +170,7 @@ impl HwpWriter {
     }
 
     /// Create a table builder for advanced table creation
-    pub fn add_table(&mut self, rows: u32, cols: u32) -> style::TableBuilder {
+    pub fn add_table(&mut self, rows: u32, cols: u32) -> style::TableBuilder<'_> {
         style::TableBuilder::new(self, rows, cols)
     }