fix(mdx): default outDir to .docs and align all templates/examples

The plugin default outDir was 'docs' while every template and example
tsconfig aliased collections/* -> ./.source/*, so a fresh fork could not
resolve `import { docs } from 'collections/server'` without manually
passing createMDX({ outDir }). Standardize on .docs everywhere:

- packages/mdx/src/core.ts: _Defaults.outDir 'docs' -> '.docs'
- 31 template/example tsconfig: collections/* -> ./.docs/*
- align generated-dir .gitignore entries to .docs
- changeset (patch)

examples/python/tsconfig.json

@@ -15,7 +15,7 @@
     "jsx": "react-jsx",
     "incremental": true,
     "paths": {
-      "collections/*": ["./.source/*"],
+      "collections/*": ["./.docs/*"],
       "@/*": ["./*"]
     },
     "plugins": [