docs: eth_subscribe api, misc. cleanup

[wolovim]

What was wrong?

• eth_subscribe and eth_unsubscribe not yet listed in API; this is a simple v1 that points to the persistent connection provider docs for usage examples.
• bonus: shuffling around the contract examples a bit
• bonus: make a transaction type warning more generic and moves it to the Troubleshooting guide
• bonus: update a reference to more recent WebSocketProvider blog post

Todo:

• Clean up commit history
• Add or update documentation related to these changes
• Add entry to the release notes

Cute Animal Picture

[fselmo]

Changes look good to me 👍🏼... nothing blocking at all, just food for thought.