Add MacOS installation instructions and update to v0.0.11

[Copilot]

README only documented Linux ARM64 installation. Added complete platform coverage for latest release.

Changes

• Linux: Added AMD64 alongside existing ARM64
• MacOS: Added Intel (AMD64) and Apple Silicon (ARM64) variants
• Version: Updated all references from mixed versions to v0.0.11 (latest release)
• Navigation: Added release page link and platform-specific subsections

Installation now covers

# Linux AMD64
curl -fsL -o /usr/local/bin/slack https://github.com/kitproj/slack-cli/releases/download/v0.0.11/slack_v0.0.11_linux_amd64

# MacOS Intel
curl -fsL -o /usr/local/bin/slack https://github.com/kitproj/slack-cli/releases/download/v0.0.11/slack_v0.0.11_darwin_amd64

# MacOS Apple Silicon
curl -fsL -o /usr/local/bin/slack https://github.com/kitproj/slack-cli/releases/download/v0.0.11/slack_v0.0.11_darwin_arm64

Original prompt

Make sure the README has instructions to install on both Linux and MacOS, and the version is the latest released version.

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