chore: improve READMEs and update recipe versions

[marc0olo]

Summary

• Rewrites the main README with a cleaner structure, getting started section, and a table linking to each template
• Fixes a critical bug where icp network run --background was advertised (command doesn't exist), replaced with icp network start -d
• Updates recipe versions to latest: motoko v4.1.0, rust v3.1.0, asset-canister v2.1.0
• Removes unnecessary args: --incremental-gc from motoko recipe configs (no longer needed in v4.1.0 and default of moc)
• Generalizes hardcoded recipe versions in README examples to @<version> with links to the icp-cli-recipes releases page
• Fixes inaccurate Key Components sections (wrong type values, wrong field names)
• Fixes broken link to backend.did and .gitignore files
• Fixes grammar issues, inconsistent heading capitalization, and missing/wrong code block language hints