docs: tighten public copy, add comparison, templates, and social preview

* docs: tighten README, CONTRIBUTING, and CLA prose

* docs: add platform comparison and README FAQ

* chore: add issue and pull request templates

* chore: add repository social preview image

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1,8 @@
+blank_issues_enabled: false
+contact_links:
+  - name: Security vulnerability
+    url: https://github.com/TheZwiss/backspace/security/advisories/new
+    about: Do not open a public issue for security problems. Report privately here.
+  - name: Question or setup help
+    url: https://github.com/TheZwiss/backspace/discussions
+    about: For usage questions, deployment help, and general discussion.