Re: doc: add missing "id" attributes to extension packaging page

As a person who had to re-rebase a patch because I discovered that id tags had been added to one of the files I was working on, I can confidently say "don't worry". It wasn't that big of a deal, I wasn't even following this thread at the time and I immediately figured out what had happened and what was expected of me. So it isn't that much of an inconvenience. If there is a negative consequence to this change, it would be that it might incentivize patch writers to omit documentation completely at the early stages. But I don't think that's a problem because people generally see a lack of documentation as a clue that maybe the patch isn't ready to be reviewed, and this change would only reinforce that litmus test.

While I can foresee people overlooking the docs build, such oversights won't go long before being caught, and the fix is simple.  Now if we can just get a threaded version of xlstproc to make the builds faster...

p.s. I'm "Team Paperclip" when it comes to the link hint, but let's get the feature in first and worry about the right character later.