docs: add cli.nylas.com website link across project

[qasim-nylas]

Summary

Adds the documentation site URL (https://cli.nylas.com/) across all key touchpoints so users and contributors can easily discover it.

Changes

File	What was added
README.md	Website badge (docs-cli.nylas.com-blue) + "Documentation" link in the header
docs/INDEX.md	Website callout at the top of the docs hub
internal/cli/root.go	Documentation: https://cli.nylas.com/ footer in nylas --help output
.goreleaser.yml	Docs link in GitHub release header + updated Homebrew formula homepage (commented out, ready for re-enable)

Additionally, the GitHub repo "About" homepage was updated to https://cli.nylas.com/ via gh repo edit.

Why

The site was live but not referenced anywhere in the project. These changes ensure visibility across:

• GitHub visitors — README badge + repo About section
• CLI users — nylas --help output
• Release consumers — GitHub release notes
• Contributors — docs index

Test plan

• go build ./cmd/nylas/ passes
• Verify README badge renders correctly on GitHub
• Verify nylas --help shows the documentation URL at the bottom