docs: slim README down to the essentials and move the rest to the docs site (#401)

The README had grown to ~700 lines and was duplicating entire sections of the
docs site (install variants, every keybinding profile, the full interactive
key list, the shell-completion recipe for three shells, the directory tree,
alias grammar, tmux notes). Now that the docs site exists and is fleshed out,
the README can just point at it.

README changes:
* Drop the split Installation section (Pre-compiled / Homebrew / Quick
  install / make build / Development setup / go install). Replace with a
  3-line quick-install snippet plus a link to the install guide.
* Drop the whole "Interactive Command Selection" section, the key tables,
  and the workflow explainer. One short paragraph links to the interactive
  guide.
* Drop the 140-line "Interactive Mode Keybindings" configuration section
  (profiles, per-OS, per-context, per-terminal blocks, config management
  examples). Keep a 6-line summary that links to the config guide.
* Drop the "Directory Structure" tree to a 6-line summary.
* Drop the per-shell "Enabling Completions" snippets; link to the install
  guide's completions section instead.
* Drop the "Supported Environments" heading — folded into Features bullets.
* Keep what only makes sense on the repo front page: badges, logo, demo GIFs,
  the command reference table, unified-syntax note, aliases intro, references,
  contributing / sponsor / stargazers.
* Normalize top-level headings to `##` (previous mix of `#` and `##` broke
  the sidebar TOC on GitHub).

Net: 709 → 218 lines.

Doc site fixes discovered while auditing:
* `docs/guide/install.md`: remove the phantom `ggc completion <shell>` flow.
  ggc does not ship that subcommand; completions live in `tools/completions/`
  and are sourced directly. Also delete a stray second copy of the page that
  was left behind when `install.md` was extended.
* `docs/guide/config.md`: delete a stray second copy of the page that was
  left behind when keybindings / profiles / layered overrides were added.
* `docs/guide/interactive.md`: fix the Workflow-mode hotkeys. The previous
  text said Ctrl+W / Enter / Esc / Ctrl+X, which don't match the default
  profile. Actual defaults: Ctrl+T toggles workflow view, `x` runs, `n`
  creates, `d`/Ctrl+D deletes, Ctrl+N/Ctrl+P cycles, `c` clears.

Author: bmf-san