x/pkgsite: add GetDocumentation function

As part of the new data model migration (#39629), we need to add a function to fetch documentation for a given path_id.

The function signature will be:
```
GetDocumentation(ctx context.Context, pathID int)(*internal.Documentation, error)
```

The function will execute the query below:
```
SELECT
  goos
  goarch
  synopsis
  html
FROM
  documentation 
WHERE
  path_id = $1
```