Rollup merge of #142736 - lolbinarycat:rustdoc-issue-template, r=GuillaumeGomez,fmease

add issue template for rustdoc

~~This also expands the scope of the "diagnostic
issue" template to include rustdoc lints,
meaning diagnostic issues will need triaging again. I think this is preferable to the alternative of
cramming even more cases under a single issue template.~~

r? t-rustdoc

Author: Kobzol