Improve documentation

[danieljanes]

Documentation improvements:

• Remove private flwr.typing module from API ref
• Migrate changelog from .rst to .md to make it easier to C&P release notes to GitHub releases. This also adds general support for MarkDown-based documentation, so any other piece of documentation could use it as well.
• Migrate from (unmaintained) sphinx_panels to the successor sphinx_design to fix FAQ incompatibility with Furo dark theme
• Remove Legacy Examples from top-level navigation
• Remove Python 3.6 references
• Update docs-related dependencies
• Update title of some docs to make the index easier to parse for new users
• Add Join us on Slack button to the docs landing page