feat(infra): resolve directory paths of the site to index.html

Closes #11 (closed). Found while proving the publish in docs!4 (merged): the bucket is filled, /index.html answers 200, / and /adr/ answer 404 because R2 does not resolve directory paths. A transform rule on the zone appends index.html to paths of the site that end in a slash.

The plan adds one resource, but the provider's dry run was refused with 403: the token in .env.infra needs the zone permission Transform Rules: Edit on corvus.observer. After adding it:

cd <root>/worktrees/platform/site_index && ahoy infra plan
cd <root>/worktrees/platform/site_index && ahoy infra apply

Then https://the.corvus.observer/ and https://the.corvus.observer/adr/ should answer 200.