Update public docs for preview period

[JakeDawkins]

• Our docs have some sections for (and other usage of) the introspect command which will not land before the public preview. Rather than rewriting any of those sections to remove the command, I noted everywhere where it was used.
• Filled in some documentation on the essentials page that was missing
• Added a section describing the public preview period and how it works

[StephenBarlow]

These changes look good! One thing @JakeDawkins: can we put each markdown paragraph on a single line? This is our convention across other docsets, as it improves GitHub diff clarity when text is edited

[JakeDawkins]

These changes look good! One thing @JakeDawkins: can we put each markdown paragraph on a single line? This is our convention across other docsets, as it improves GitHub diff clarity when text is edited

for sure! I for some reason was under the impression diffs were easier on multiple lines, but after thinking about it, a single line makes more sense! 😄