Remove containers; revamp network & storage providers

[mweidner037]

• Completes https://github.com/composablesys/collabs/issues/253 by removing containers.
• Completes https://github.com/composablesys/collabs/issues/241 by rewriting @collabs/ws-client and @collabs/ws-server, and adding @collabs/indexeddb, @collabs/local-storage, and @collabs/tab-sync. I have not revised @collabs/matrix-widget (that's https://github.com/composablesys/collabs/issues/209) or re-added a WebRTC network.
• Makes some tweaks to how CRuntime handles updates (in particular, the "Update" event in place of "Transaction") to make it easier to mesh providers. Also exposes vector clocks / causal dots to make it easier to manage at-least-once delivery yourself.
• Rewrites demos to use @collabs/ws-client instead of containers, and rewrites template-app to use various providers.

Still needs:

• [x] Test providers
• [x] Some API docs and TODOs
• [x] Rewrite guide to use these providers instead of containers.

[mweidner037]

I only meant to remove the container references from the guide, but ended up rewriting a lot of it:

• New "Documents" page to replace old "Initialization"
• "Providers" and "Updates & Sync" pages
• Rewrite misc content that was outdated or overly verbose (including the landing page)