Add practical examples to abilities tutorial

[anovstrup]

This PR fleshes out the Abort/Exception section with a practical validation example and adds a section for the Store ability and a Memo memoization ability.

[netlify[bot]]

Deploy request for unisonweb-org accepted.

Accepted with commit 079b8433fd9d8149034a033ac28fefd31531182e

https://app.netlify.com/sites/unisonweb-org/deploys/5e9a307c07dc6f0007d28e30

[anovstrup]

@pchiusano I'd appreciate feedback on this PR and #113 when you have a chance. 🙏

[anovstrup]

I notice that netlify is no longer automatically deploying a preview of the site with the changes, which is unfortunate as it makes self-review more difficult. :(

[pchiusano]

This is on my list to review, sorry for the delay!

[aryairani]

I wonder if it does make sense.  Wouldn't you want to preview a site before approving a PR? 🤪

[anovstrup]

@pchiusano Bump.