chore(github): add detailed issue & PR templates, capitalize title prefixes

Adds five new issue templates (bug_report_detailed, feature_request_detailed,
performance_regression, documentation, question) alongside the existing
short forms, plus an optional detailed PR template under
.github/PULL_REQUEST_TEMPLATE/ that contributors can opt into via the
?template=detailed.md URL. Existing templates keep their behavior; only
title prefixes and prose-paren wording were capitalized for consistency.

.github/ISSUE_TEMPLATE/documentation.md

@@ -0,0 +1,33 @@
+---
+name: Documentation issue
+about: Something in the README, rustdoc, examples, or guides is wrong, missing, or confusing.
+title: "[Docs] <short description>"
+labels: ["documentation", "good first issue"]
+assignees: []
+---
+
+## Where
+
+<!-- Link or path. e.g. README.md#streaming-vs-batch, docs/guide/ema.md, rustdoc for `wickra::Ema::update`. -->
+
+## What's wrong / missing
+
+<!--
+- [ ] Incorrect information
+- [ ] Outdated for current API
+- [ ] Missing example
+- [ ] Unclear wording
+- [ ] Broken link / broken code block
+- [ ] Other
+-->
+
+## Suggested change
+
+<!--
+Paste the corrected wording, a clearer example, or a sketch of the
+section you'd like to see. PRs welcome.
+-->
+
+## Additional context
+
+<!-- Quote of the confusing passage, screenshot, etc. -->