docs: rewrite README as a slim modernized front-door

Replaced the 1049-line README.md (which inlined the entire user
manual) with a ~227-line README.org that's the standard 'GitHub
front page' for an Emacs package.  The deep manual lives where it
already does — in org-drill.org, which is also a runnable demo
deck.

Sections, in order:
- Title + nav links
- Five badges: made-for-emacs, MELPA, MELPA Stable, GitHub Actions
  CI, Coveralls.  CI and coverage badges are aspirational until
  the GitHub Actions TODO lands; their URLs are correct so they
  auto-populate when CI is set up.
- Maintenance status: explains the fork, lists recently-fixed
  upstream issues, points at the GitHub mirror and Issues.
- Features: bullet list of capabilities
- Installation: MELPA, package-vc-install, use-package :vc,
  Straight, manual.  Drops the stale 'tick drill in org-modules'
  + Org contrib references.
- Quick Start: 60-second flow from install to first drill
- In-Session Keys: cheat sheet table for both the question and
  rating phases (closes the existing #A TODO for the cheat sheet)
- Configuration: most-used defcustoms with comments
- Development: make-target table
- History: contrib → phillord → cjennings handoff story
- License pointer

Closes three #A TODOs: README modernization, stale install
instructions, in-session keybinding cheat sheet.

Added assets/made-for-emacs-badge.svg (copied from emacs-wttrin).

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