docs: add feature request + improvement issue templates, auto-assign new issues (#21)

- Add 2-feature_request.md and 3-improvement.md templates (chooser now
  lists Bug report, Feature request, Improvement in order)
- Improve bug report template: cross-template pointer, Linux log hint,
  Related issues/PRs section, demo-DB reproduction hint
- Auto-assign every newly opened issue to the repo owner:
  assignees in template front matter + a workflows/auto-assign.yml
  (covers issues opened without a template)
- Add ISSUE_TEMPLATE/config.yml with Roadmap/README contact links
- Update README Contributing section

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,10 @@
+# Template chooser settings — see
+# https://docs.github.com/communities/using-templates-to-encourage-useful-issues-and-pull-requests/configuring-issue-templates-for-your-repository
+blank_issues_enabled: true
+contact_links:
+  - name: Gridline Roadmap
+    url: https://github.com/AdrianBonpin/gridline/blob/prod/ROADMAP.md
+    about: Check what's already planned before requesting a feature
+  - name: README — feature list
+    url: https://github.com/AdrianBonpin/gridline#features
+    about: Confirm a feature isn't already shipped