SvelteKit powered docs site + refresh brand guidelines

[abovedave]

Replaces Eleventy for a SvelteKit powered docs site:

• New docs can be added as .md files in packages/stacks-docs/src/docs/
• For complex documentation requiring images and interactivity you can also create a folder with an index.md. See /packages/stacks-docs/src/docs/public/brand/typography/ as an example.
• Navigation is updated by editing packages/stacks-docs/src/structure.yaml
• ⚠️ In-progress: Migrate old stacks documentation to svelte powered, using stacks-svelte where appropriate. This can be ongoing work as we build out new sections.
• Upgraded Agolia DocSearch, although now setup with the new structure yet.

To get started:

npm i
npm run dev -w packages/stacks-site

Adds support for gated documentation for employees, this is useful for non-technical folks finding brand or design related information e.g., location of internal presentation decks, brand strategy details.

Employees will need access to StackEng/StacksDocsPrivate and SSH authentication setup when cloning.

git clone -b beta https://github.com/StackExchange/Stacks.git
cd ./Stacks
git submodule update --init --recursive

Change beta to new-ui to test this branch

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Latest commit: 7917691f66dfb10a09bbf41f75dc7141ce7f1218

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[abovedave]

Moving this work to alpha branch, will reopen a PR when we are closed to a switchover