docs: improve XY README for Python graphing and comparison queries
The README did not explicitly describe XY as a Python graphing library or answer common library-comparison questions. This update adds a descriptive H1, a sourced Matplotlib/Plotly/Bokeh comparison, a same-size scatter benchmark summary, and nine FAQs covering migration, performance, styling, exports, chart families, and alpha status.
The benchmark summary separates density aggregation from exact markers, identifies the first-party machine/run scope, and marks standalone Bokeh as unmeasured. It also avoids treating the 258 KiB binary density payload as an HTML export. Adds dynamic stars/version badges and removes the already-shipped funnel chart from the roadmap. Existing code blocks, diagrams, image URLs, and benchmark tables are preserved.
Validation: full pre-commit hooks, Ruff lint and format checks, and git diff --check pass. Verified local links/anchors, all 10 original fenced blocks and original image references, GitHub Markdown rendering, and the comparison table in GitHub's browser preview. Documentation-only change; benchmarks were not rerun.