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

docs/content/en/functions/RenderString.md

@@ -0,0 +1,37 @@
+---
+title: .RenderString
+description: "Renders markup to HTML."
+godocref:
+date: 2019-12-18
+categories: [functions]
+menu:
+  docs:
+    parent: "functions"
+keywords: [markdown,goldmark,render]
+signature: [".RenderString MARKUP"]
+---
+
+{{< new-in "0.62.0" >}} 
+
+`.RenderString` is a method on `Page` that renders some markup to HTML using the content renderer defined for that page (if not set in the options).
+
+The method takes an optional map argument with these options:
+
+display ("inline")
+: `inline` or `block`. If `inline` (default), surrounding ´<p></p>` on short snippets will be trimmed.
+
+markup (defaults to the Page's markup)
+: See identifiers in [List of content formats](/content-management/formats/#list-of-content-formats).
+
+Some examples:
+
+```go-html-template
+{{ $optBlock := dict "display" "block" }}
+{{ $optOrg := dict "markup" "org" }}
+{{ "**Bold Markdown**" | $p.RenderString }}
+{{  "**Bold Block Markdown**" | $p.RenderString  $optBlock }}
+{{  "/italic org mode/" | $p.RenderString  $optOrg }}:REND
+```
+
+
+**Note** that this method is more powerful than the similar [markdownify](functions/markdownify/) function as it also supports [Render Hooks](/getting-started/configuration-markup/#markdown-render-hooks) and it has options to render other markup formats.