Change markdown rendering from blackfriday to goldmark (#9533)

* Move to goldmark

Markdown rendering moved from blackfriday to the goldmark.

Multiple subtle changes required to the goldmark extensions to keep
current rendering and defaults.

Can go further with goldmark linkify and have this work within markdown
rendering making the link processor unnecessary.

Need to think about how to go about allowing extensions - at present it
seems that these would be hard to do without recompilation.

* linter fixes

Co-authored-by: Lauris BH <lauris@nix.lv>

vendor/github.com/shurcooL/sanitized_anchor_name/main.go

@@ -1,29 +0,0 @@
-// Package sanitized_anchor_name provides a func to create sanitized anchor names.
-//
-// Its logic can be reused by multiple packages to create interoperable anchor names
-// and links to those anchors.
-//
-// At this time, it does not try to ensure that generated anchor names
-// are unique, that responsibility falls on the caller.
-package sanitized_anchor_name // import "github.com/shurcooL/sanitized_anchor_name"
-
-import "unicode"
-
-// Create returns a sanitized anchor name for the given text.
-func Create(text string) string {
-	var anchorName []rune
-	var futureDash = false
-	for _, r := range text {
-		switch {
-		case unicode.IsLetter(r) || unicode.IsNumber(r):
-			if futureDash && len(anchorName) > 0 {
-				anchorName = append(anchorName, '-')
-			}
-			futureDash = false
-			anchorName = append(anchorName, unicode.ToLower(r))
-		default:
-			futureDash = true
-		}
-	}
-	return string(anchorName)
-}