fix: strip trailing whitespace from generated vimdoc

[geodimm]

Generated vimdoc has trailing whitespace. Code blocks indent their blank lines, lists and definition lists indent them again, and tables pad their cells. Editors and pre-commit hooks that trim whitespace then rewrite the file after every run, so it never matches fresh output (which also defeats #77).

This strips trailing whitespace from the rendered body once, in Writer.Pandoc.

For example, this Markdown:

local a = 1

local b = 2

renders as follows. $ marks the end of each line:

 >lua$
     local a = 1$
-    $
+$
     local b = 2$
 <$

• An empty line doesn't end a code block in Vim's or Neovim's help syntax, or in Neovim's tree-sitter parser. Neovim's own docs use empty lines inside code blocks.
• The fix sits in Writer.Pandoc rather than CodeBlock because lists and definition lists skip empty lines, so blank lines in nested code blocks would otherwise disappear.
• The render tests stripped trailing whitespace before comparing, which hid this; they now compare exact output. A new fixture covers blank lines in code blocks at the top level and in a list.