T21a: Documents, editing

- markdown read with pulldown-cmark: marks for live styling, blocks, links, words
- forms::Highlight gains weight, slant and strike; measure and paint share runs
- Inter Italic embedded, so emphasis reads slanted
- forms::mention's popup and keys become Suggestions, shared with the editors
- RichTextEditor with FixedFormatToolbar, the floating toolbar, LinkEditor,
  @ and : suggestions, WordCount and ReadingTime
- MarkdownEditor: visual (a pressed block opens as source), source, split
- BlockEditor: prose, lists, to-dos, quotes, callouts, toggles, code, math,
  diagrams the app draws, dividers, tables that grow and merge, pictures to
  resize and align, cards for video and pages, columns, synced text; slash menu,
  markdown shortcuts, handles that drag and open a menu, one undo for all
- TodoItem and Checklist
- icons: heading-1..3, workflow, merge
- gallery: the Documents page with scripted states

examples/gallery/pages/mod.rs

@@ -2,6 +2,7 @@ mod buttons;
 mod charts;
 mod data;
 mod debug;
+mod documents;
 mod editor;
 mod feedback;
 mod finance;
@@ -55,6 +56,7 @@ pub const ALL: &[Page] = &[
     terminal::PAGE,
     git::PAGE,
     debug::PAGE,
+    documents::PAGE,
     theme::PAGE,
 ];