docs: documentation for at() and get() methods

[jemgillam]

Description

Needs review!

• detail on implementing at and get added under the header "Access methods"
• added missing detail about the methods a ListStep can accept
• Saw a missing link while reviewing the methods for a Lambda step

Closes graphile/crystal-pre-merge#497

[changeset-bot]

⚠️ No Changeset found

Latest commit: bc03aec

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[benjie]

Really good improvements! Here's some suggested edits 🙌

[jemgillam]

I've reorgainsed the sections on the step-classes page as discussed

I've added a blurb to the https://grafast.org/grafast/step-library/standard-steps/ page as it is linked to directly from the step-classes page, I wrote a small summary taken from the step-library category page