Move "writing documentation" page into Contributing (#6789)

While we very much want to introduce an "explanation" section in some form, I don't think this should be the inaugural item in that section, as it's 'meta' documentation about the Wagtail documentation process, rather than information directed at Wagtail site creators.

docs/contributing/index.rst

@@ -66,5 +66,6 @@ More information
     css_guidelines
     javascript_guidelines
     documentation_guidelines
+    documentation-modes
     security
     release_process

docs/explanation/index.rst

@@ -1,8 +0,0 @@
-===========
-Explanation
-===========
-
-.. toctree::
-    :maxdepth: 2
-
-    documentation-modes

docs/index.rst

@@ -39,7 +39,6 @@ Index
    topics/index
    advanced_topics/index
    reference/index
-   explanation/index
    support
    editor_manual/index
    contributing/index