Improve frame-omni-bencher docs

[iulianbarbu]

Straight-forward changes:

• the command from the usage example should be backed by integration tests, as we did in staging-chain-spec-builder, and exported to the docs by using docify::export_content
• the README.md should be generated from a README.docify.md as we did with staging-chain-spec-builder again, by running a cargo command with --features generate-readme.
• the README.md should be include_stred into a lib.rs (even if the crate is intended to be a binary create) - currently the README's content is duplicated as public docs for command.rs::Command struct, which isn't displayed/used anywhere. docs.rs will show something like below (which I assume is taken from crates.io):

Not a must, but good practice related, as we did with staging-chain-spec-builder:

• existing integration tests could use cmd_lib::run_cmd or other cmd_lib alternative, based on the assertion logic needs.
• we might extend the usage examples with some more representative commands, backed by the above integration tests, if really useful (I am not very familiar with frame-omni-bencher usage, but we'll want to clarify that at some point).

[rainbow-promise]

Let me take a crack at it @iulianbarbu

[iulianbarbu]

Thanks @rainbow-promise , go for it. Assigned this to you.

[iulianbarbu]

hey @rainbow-promise ! How is it going? Please let us know if you encounter blockers with this, and no worries if you haven't got a chance to start it, just thought about checking in.

[rainbow-promise]

Hey @iulianbarbu, currently working on this #6321

Should have a PR by the week and thanks for checking in 😁

[iulianbarbu]

Hey @iulianbarbu, currently working on this #6321

Should have a PR by the week and thanks for checking in 😁

Amazing, take your time. Looking forward to the PR when it's ready. 😄