docs: Helia providing content

We had a user reach out to us on slack about their content not being provided in the DHT. They were unaware that Helia does not provide content by default.

This is a different experience than Kubo, and we have no documentation mentioning this requirement.

What can we do:

1. update docs to mention what expectations should be
2. update Helia 101 example with providing steps
3. Automatically provide when content is added
4. Clarify when a user should understand "provide" to be completed
* What does it mean that the promise from `.provide` resolves?
* Should users be checking for how many `FINAL_PEER` events they get?