spec: headings should include anchor links

[mvdan]

Take a look at https://pkg.go.dev/io#Pipe. When hovering the heading for the declaration, you can see a "¶" symbol, which makes it very easy to get a link anchored to that part of the page.

The Go spec lacks this. If I want an anchored link to https://go.dev/ref/spec#Type_definitions, I have two options:

1. Go to the index to find the link and copy it. This does not always work - some sub-headings like "Type definitions" are not in the index.
2. Use my browser's "Inspect" feature to fetch the header ID so I can manually construct the anchored link - which I did in this case.

It would be very helpful to provide "¶" symbols when hovering, much like pkgsite does, in my opinion.

cc @griesemer and cc @julieqiu in case any of pkgsite's html/css can be reused for this purpose

[mvdan]

@rogpeppe and @myitcv remind me that we've also wanted anchor links on hover for other structured documentation pages under go.dev, such as https://go.dev/ref/mod - cc @bcmills

[ALTree]

See #50210

[mvdan]

Ah, thank you, I completely missed that one. I will instead leave a comment there.