feat: add project infrastructure and rewrite README

Problem: nonicons.nvim lacks CI, pre-commit hooks, nix dev shell, issue
templates, and the README contains content that duplicates the vimdoc.

Solution: add quality/luarocks CI workflows, pre-commit config, nix
flake, prettier config, GitHub issue/discussion templates, and rewrite
the README to minimal style with all detailed content in vimdoc. Update
yamatsum/nonicons references to ya2s/nonicons.

.github/DISCUSSION_TEMPLATE/q-a.yaml

@@ -0,0 +1,17 @@
+title: 'Q&A'
+labels: []
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Use this space for questions, ideas, and general discussion about nonicons.nvim.
+        For bug reports, please [open an issue](https://github.com/barrettruth/nonicons.nvim/issues/new/choose) instead.
+  - type: textarea
+    attributes:
+      label: Question or topic
+    validations:
+      required: true
+  - type: textarea
+    attributes:
+      label: Context
+      description: Any relevant details (Neovim version, config, screenshots)