add contribution docs

Author: chargome

docs/contributing/pages/placeholders.mdx

@@ -0,0 +1,12 @@
+---
+title: Placeholders
+noindex: true
+---
+
+
+The following placeholders are available in code blocks and will be replaced at build time:
+
+- `___JS_PACKAGE___`: The current JavaScript package. (e.g. `@sentry/react`)
+- `___PUBLIC_DSN___`: The public DSN for a Sentry project. (will display a dropdown of available projects if the user is logged in)
+
+All placeholders are wrapped with **three underscores**.