chore: remove Mintlify template leftovers and update README

[dangrondahl]

Summary

• Removes 12 files carried over from the Mintlify starter template that are not referenced in docs.json and contain no Kosli content
• Rewrites README.md to reflect the actual repo purpose, dev workflow, deployment process, and automation

Deleted files

File/directory	Reason
essentials/ (6 files)	Mintlify's own authoring guide — flagged in CLAUDE.md as do-not-edit
development.mdx	Mintlify's "how to use Mintlify" page
quickstart.mdx	Mintlify's generic quickstart page
ai-tools/ (3 files)	Generic "configure AI tool for your docs workflow" pages
api-reference/introduction.mdx	Template filler referencing "Plant Store Endpoints"

README changes

• Added [!WARNING] callout that this is not yet the official documentation
• Replaced generic Mintlify boilerplate with Kosli-specific content
• Documents dev commands (mint dev, mint broken-links, mint a11y, mint update)
• Notes that opening a PR creates a Mintlify preview environment
• Documents the .mintlify/workflows/update-changelog.md automation

Test plan

• mint broken-links passes (no new broken links introduced)
• Preview environment renders correctly without the removed pages