[readme generation] Generate readmes with code snippets, or appropriate snippet tags

Each language has a unique way of including snippets in their readmes. For most of these we just need to insert an appropriate slug, based on an example they already have.

This one has some tie-in to the samples generator since samples and snippets (in some languages) overlap, so we'll want to do this _after_ we figure out how that works.

Start by understanding how the samples generator works and review the existing language-specific README examples that use snippets or snippet tags. After the samples/snippets overlap is resolved, done means generated READMEs include the appropriate code snippets or tags for each language.

Written by the indexing model from the issue text.