Removed content from architecture overview

[armiller123]

Removed content from architecture overview that we're not doing anymore.

Also updated title

[armiller123]

Specific updates:

• Removed "Modular Architecture" section
• Removed "Confidentiality" section
• Moved "Decentralized Consensus Layer" and Parallel runtime sections to top
• Moved "Definitions" section to the end

[peterjgilbert]

LGTM, but it's failing some lints. Want me to try to fix?