Restructure the README

[avgvi]

The README opened with a long paragraph about dependency pins and kept the main visual and the reasons to use the project far down a 343-line page. This restructures it so the first screen says what the project does and why to use it, and the rest scans as tables.

• Top unchanged: logo, title, tagline, Website, Quick Start and Docs links, and all badges.
• New pitch line and a jump-link row (Install, Quick start, Why this fork, Features, Storage, Migrating, Docs), with flow.gif moved up as the hero image.
• "Why streaming-community": five bullets, each backed by CI or a merged pull request.
• Installation: one command and a table of the optional extras.
• Quick start: one short example that writes MDS shards and streams them through a DataLoader. Unlike the previous examples, which were all skipped, it runs under pytest-codeblocks in CI.
• Key features: one table with a one-line benefit per feature, each linking to its docs page. The mixing example and the determinism, convergence and throughput results move into collapsible sections; the results are labelled as measured for the original MosaicML Streaming.
• New tables for storage backends (address prefixes and required extras, taken from streaming/base/storage/download.py) and for compatibility (supported ranges next to what CI checks).
• About this fork: the fork paragraph is unchanged; "What stays the same" and the migration steps are unchanged inside a collapsible section. The starter-code table, the project showcase and the citation are collapsed too.
• Community section with Discussions, Issues, CONTRIBUTING, SECURITY and CHANGELOG, and the star line.
• Fixes the "Disk usage limits" link, which pointed at a docs file that does not exist.

Only HTML that PyPI keeps is used (tables, <details>, <img>, <div align>), and all images use absolute URLs. The jump links target the heading ids GitHub and PyPI generate.

Checked: pytest README.md gives 1 passed (the quick start) and 6 skipped; the pre-commit hooks pass; twine check --strict passes on a built wheel; on the pushed branch, GitHub renders all 6 collapsible sections and 6 tables, and every jump link resolves to its section. The PyPI page changes with the next release.