`create-spectacle` CLI: Fix generated spectacle version, add custom types for MD build.

[carloskelly13]

• Clean up the TypeScript index.tsx template file. This will allow us to expand the content to support new outputs down the road. Also fixes the formatting so it's nicely indented.
• Fix the package.json version of spectacle to not be a * which trips up pnpm and uses the version from the core library's package.json
• Creates a custom.d.ts file in the Markdown output so the import of the .md file does not throw an unknown type error.

Screenshot of the cleaned up, correctly indented index.tsx:

How to test

1. cd to packages/create-spectacle
2. run pnpm dev
3. Try out md and the two React flavors
4. npm i && npm run start in each output and verify they load up

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Name	Status	Preview	Comments	Updated (UTC)
spectacle-docs-website	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Dec 28, 2023 3:10pm