update auto_identifier extension to gfm_auto_identifier

[zkamvar]

The following markdown produces an invalid link:

## 1\. introduction

[link to intro](#1-introduction)

It looks like pandoc's auto_identifiers extension strips the leading number from numbered sections (emphasis mine):

The default algorithm used to derive the identifier from the heading text is:

• Remove all formatting, links, etc.
• Remove all footnotes.
• Remove all non-alphanumeric characters, except underscores, hyphens, and periods.
• Replace all spaces and newlines with hyphens.
• Convert all alphabetic characters to lowercase.
• Remove everything up to the first letter (identifiers may not begin with a number or punctuation mark).
• If nothing is left after this, use the identifier section.

I believe the solution is for me to update {sandpaper} to use the gfm_auto_identifiers extension instead, which should be updated here:

sandpaper/R/render_html.R

Line 67 in 3bb22ef

"auto_identifiers",

As for why #1-introduction is being transformed to #id_1-introduction, that's a bit of a mystery that I have to dig further into.

Originally posted by @zkamvar in carpentries/instructor-training#1497 (comment)