[docs] Align documentation style guide and update skill

Update documentation-style-guide.md to adhere to the edit-fuchsia-doc
skill guidelines, including active voice, sentence case headings, and
dash-based anchors.

Format example <pre><code> code blocks so both opening and closing
backticks start at Column 0, keeping Hoedown Markdown engine code
block toggling balanced.

Bug: None
Test: fx doc-preview docs/contribute/docs/documentation-style-guide.md
Change-Id: Ia5c49a55f7770b45fee1d8169349ec97318cf478
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1742229
Commit-Queue: Kyo Lee <kyol@google.com>
Reviewed-by: Nick Van der Auwermeulen <nickvander@google.com>