Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

markup/rst/convert.go

@@ -19,6 +19,7 @@ import (
 	"os/exec"
 	"runtime"
 
+	"github.com/gohugoio/hugo/identity"
 	"github.com/gohugoio/hugo/markup/internal"
 
 	"github.com/gohugoio/hugo/markup/converter"
@@ -48,6 +49,10 @@ func (c *rstConverter) Convert(ctx converter.RenderContext) (converter.Result, e
 	return converter.Bytes(c.getRstContent(ctx.Src, c.ctx)), nil
 }
 
+func (c *rstConverter) Supports(feature identity.Identity) bool {
+	return false
+}
+
 // getRstContent calls the Python script rst2html as an external helper
 // to convert reStructuredText content to HTML.
 func (c *rstConverter) getRstContent(src []byte, ctx converter.DocumentContext) []byte {