Theme scope cleanup

[smartinellimarco]

Cleans up theme scopes that drifted between the code and the book.

• The active tab falls back to ui.statusline and the background to ui.statusline.inactive, and ui.statusline.active is no longer read, as suggested in add ui.statusline.active documentation #9567 (comment).
  github_dark and github_light move their ui.statusline.active to ui.bufferline.active so they render the same.

• Removes markup.normal, markup.normal.{completion,hover,raw}, markup.heading.{completion,hover,raw} and markup.raw.inline.{completion,hover} from the book and bundled themes.
  The popup namespacing was dropped in Allow separate styles for markup headings #1618 (comment) and markup.normal in e7f5ec5, but the book and themes kept them.

• Documents ui.gutter.virtual, ui.gutter.selected.virtual and ui.cursorcolumn, which the code reads but the book didn't list.

Themes that set no ui.bufferline* scope now draw the bufferline background with ui.statusline.inactive.

Refs #9239, #3649