permit `{.sref}` links to individual paragraphs

[ericniebler]

currently an sref like [exec.continues.on#5]{.sref} correctly generates a link to p5 of [exec.continues.on], but will generate a warning:

mpark/wg21: stable name exec.continues.on#5 not found

and then fail to include the subsection number. this PR fixes that.

[mpark]

Thanks for the fix @ericniebler! Do you mind also updating the doc and the test please? (Doc and Test)

The test should be pretty easy to update and regenerate TEST.html and TEST.pdf files.

For the doc, if taking the screenshot is too much of a hassle, just leave it and I can do it later. If you're willing to do it, a screenshot of the HTML version is totally fine too!