Use `white-space: pre-wrap` to preserve white space as per `pre`, but to
allow line wrapping if the display runs out of horizontal space.

This prevents lines overflowing their box, and causing horizontal
scrolling across the entire page on small screens.

This `pre-wrap` technique is used by GitHub to format code for mobile.

See https://developer.mozilla.org/en-US/docs/Web/CSS/white-space#pre-wrap

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

- https://github.com/neovim/tree-sitter-vimdoc v1.2.4 eliminates most
  errors in pi_netrw.txt, so we can remove that workaround from
  ignore_parse_error().
- improved codeblock

download from https://highlightjs.org/download/
place `highlight/` directory next to `css/`

style needs adapting for Neovim colors

There are 6 remaining 8.0.x patches, tracked in:
https://github.com/neovim/neovim/issues/5431

Remove the user-manual ToC from help.txt, because:
1. it duplicates usr_toc.txt
2. it is not what most readers are looking for in the main help page.

fix https://github.com/neovim/tree-sitter-vimdoc/issues/49
fix https://github.com/neovim/tree-sitter-vimdoc/issues/50
fix https://github.com/neovim/tree-sitter-vimdoc/issues/51

Problem:
`gen_help_html.lua` does not properly handle tab characters after
"concealed" text (tags, taglinks, codespans). This causes misaligned
layout in "old" (preformatted) docs.

For text like `*tag*`, |tag_link|, and `code_span`, Vim hides the "*",
"|", "`" characters, but Vim still counts those characters for "virtual
column" when a tab character follows it. So if you have a tag of say
6 characters long, those two concealed character would lead to the tab
character after it start at column 8. gen_help_html.lua doesn't account
for that which leads to formatting flaws in the generated output.

Solution:
Add two spaces after concealed nodes that are followed by a tab char.