Revamp try-runtime content

[kianenigma]

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Content request

paritytech/substrate#12537 (comment)

Are you willing to help with this request?

Yes!

[lisa-parity]

Relevant PR:
paritytech/substrate#12537 (comment)

The try-runtime documentation is split into two topics:

• Check runtime (what it is, why it's useful, how it works, example scenarios of how to use it)
• try-runtime command-line reference (basic syntax, command-line options, code examples)
  Currently, the content is both out of date and has redundant examples sections (the intent was to talk about the use case and why the options are used in the Check runtime topic and have one sentence stubs for command-line examples in the command-line reference section).
  It needs to be revised taking into account the latest comments in the code and new features that have been added to it.
  I think the separation makes sense, but an argument could be made for just having a more fleshed out and better written "Check runtime" topic.

@har00ga , can you take this on? Let me know if you have trouble accessing the paritytech repo to look at the code.