help-beta: Convert our custom file import syntax to astro.

We use all these markdown files in `include` only to import them into
our main help markdown files. Most of these files are bulleted lists of
text. When importing these files, they might appear as an indented
sublist of an existing list. We plan to introduce mechanism to flatten
it in further commits.

help-beta/.gitignore

@@ -13,6 +13,7 @@ yarn-error.log*
 pnpm-debug.log*
 
 *.mdx
+*.md
 
 
 # environment variables