docs: get-started guide + docs embed package

[BCook98]

What

Adds an author-facing getting-started guide and a small Go package that
embeds it so the shellcade arcade can render it directly in its "Add your
own game" screen (a glamour-rendered Markdown pane inside the 80x24 SSH
TUI).

• docs/get-started.md — the guide. Sections: what a shellcade game is,
  Quickstart (shellcade-kit new / go mod tidy / go run . / check
  / play), Publish your game (PR to the public games catalog; a merged
  PR cuts a release; an operator reviews and takes it live), Link your
  GitHub (connect over SSH and use the User menu so games attribute to
  you), and Where to go next (GUIDE.md / ABI.md).
• docs/docs.go — package docs, embeds the guide as docs.GetStarted.
• a minor changeset for the new docs package export.

Content rules

• Terminal-first: prose is hard-wrapped at <=76 columns (current max
  is 74) with no HTML, images, badges, or wide tables, so it renders
  cleanly in the pager. 70 lines total.
• Public author content only: sourced from this repo's README/GUIDE
  and the public repos github.com/shellcade/kit and
  github.com/shellcade/games — no internal references.

Notes

The Quickstart points authors to this repo's Releases for the
shellcade-kit binary, matching the install method documented in
README.md and GUIDE.md (there is no go install path published for the
tool).

go build ./... and go test ./... pass; the new file is gofmt-clean.

🤖 Generated with Claude Code

[BCook98]

Should offer brew for macOS, and this as an alternative inside an expand.

[BCook98]

Done in dc9c7d5. The Quickstart now leads with the macOS brew command:

brew install shellcade/tap/shellcade-kit

(the documented cask from the GoReleaser homebrew_casks config — its
post-install hook strips quarantine, and brew downloads skip it anyway).

The manual Releases download — and the xattr quarantine note, which is
only relevant to hand-downloaded binaries — now lives inside a <details>
expand titled "Manual download (Linux / Windows / no brew)". The <details>
/</details> tag lines sit alone between blank lines, so GitHub collapses
the block while glamour (the shellcade TUI renderer) drops the raw HTML tag
lines and still renders the inner markdown inline — terminal users in the
"Add your own game" screen lose nothing. All lines stay <=76 cols.