Split rule documentation into separate files

doc/MD007.md

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+# MD007 - Unordered list indentation
+
+Tags: bullet, ul, indentation
+
+Aliases: ul-indent
+
+Parameters: indent (number; default 2)
+
+This rule is triggered when list items are not indented by the configured
+number of spaces (default: 2).
+
+Example:
+
+    * List item
+       * Nested list item indented by 3 spaces
+
+Corrected Example:
+
+    * List item
+      * Nested list item indented by 2 spaces
+
+Rationale (2 space indent): indenting by 2 spaces allows the content of a
+nested list to be in line with the start of the content of the parent list
+when a single space is used after the list marker.
+
+Rationale (4 space indent): Same indent as code blocks, simpler for editors to
+implement. See
+<http://www.cirosantilli.com/markdown-styleguide/#indented-lists> for more
+information.
+
+In addition, this is a compatibility issue with multi-markdown parsers, which
+require a 4 space indents. See
+<http://support.markedapp.com/discussions/problems/21-sub-lists-not-indenting>
+for a description of the problem.
+
+Note: This rule applies to a sublist only if its parent lists are all also
+unordered (otherwise, extra indentation of ordered lists interferes with the
+rule).