Cardano Updates

...to reflect changes to flake.nix and default.nix.

Major Updates Applied:

1.  Quick Start Section

    + Changed `nix-shell -A devShells.ci` → `nix-shell` (uses new default)
    + Updated all example commands to reflect the new shell structure

2.  Development Environments Section

    + Completely rewrote to reflect the new three-shell system:

      + `devShells.default` (Main Development) - everyday Agda work
      + `devShells.ci` - minimal CI builds
      + `devShells.docs` - complete documentation publishing

    + Removed all references to the non-existent `devShells.mkDocs`

3.  Development Workflow Section

    + Reorganized into three clear workflows matching the three shells
    + Updated commands throughout to use correct shell names
    + Added comprehensive documentation publishing workflow

4.  Using Nix Flakes Section

    + Fixed shell references to match available shells
    + Clarified which development environments are actually available

5.  Troubleshooting Section

    + Fixed `fls-shake: command not found` description (now available in multiple shells)
    + Added flake command error troubleshooting
    + Updated tool availability information

6.  Integration with IDEs

    + Updated Emacs quick setup to use `nix-shell` (default) instead of old shell names

---

Tool Availability Corrections

| Tool                | Old Documentation   | New Reality                       |
|---------------------|---------------------|-----------------------------------|
| `fls-shake`         | Only in `ci` shell  | In both `default` and `ci` shells |
| `agda`              | Multiple shells     | Available in all three shells     |
| `mkdocs`, `mdbook`  | In `mkDocs` shell   | In `docs` shell                   |
| `cargo`, `chromium` | Not mentioned       | In `docs` shell                   |

---

Command Updates

BEFORE

```bash
nix-shell -A devShells.mkDocs  # ❌ Doesn't exist
nix-shell -A devShells.ci      # ✅ Still works
```

AFTER

```bash
nix-shell                      # ✅ Default development
nix-shell -A devShells.docs    # ✅ Documentation publishing
nix-shell -A devShells.ci      # ✅ CI builds
```

Users should no longer encounter "command not found" errors or references to non-existent shells.