docs: add anyplot.ai migration guide and rebrand design docs

Comprehensive migration plan (docs/migration-anyplot.md) covering the
full transition from pyplots.ai to anyplot.ai in 6 phases: GCP project
setup, code changes, data migration, deployment, redirect, and plot
regeneration. Includes infrastructure improvements: Artifact Registry
(replaces deprecated gcr.io), Cloudflare CDN, Workload Identity
Federation (keyless GitHub→GCP auth), and GCS via CDN.

Also adds brand guidelines, frontend design system, plot color palette
(Okabe-Ito), and landing page mockup for the anyplot.ai redesign.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: MarkusNeusinger