docs: announce base-anvil in the README and expand the testing guide

[amiecorso]

Summary

• Add a "base-anvil: Base's Foundry build" section to the top of the README, so a developer landing on the repo immediately sees what it is, the base-foundryup install (not stock foundryup), and links to the user guide (docs/base.md) and the maintainer reference (RELEASES.md). Also flags, in the inherited Installation section, that Base users should use base-foundryup.
• Expand docs/base.md into a standalone testing guide. Keeps the existing install / version / interfaces / run-tests content and adds:
  • a What is Base-specific command reference (base-* commands, --base, base = true / FOUNDRY_BASE=true, base-foundryup)
  • the local base-anvil node (seeded activation features, chain id, network table)
  • troubleshooting (the call to non-contract address / FeatureNotActivated / version-mismatch cases)
  • next steps linking to the Base docs B20 launch guide and the B20 standard

Only the Base delta is documented; the doc links to getfoundry.sh for the standard forge/cast/anvil/chisel reference rather than forking upstream docs (avoids drift). Intended to be linkable from the Base docs site.