omni-node: --dev sets manual seal and allows --chain to be set (#6646)

# Description

This PR changes a few things:
* `--dev` flag will not conflict with `--chain` anymore, but if
`--chain` is not given will set `--chain=dev`.
* `--dev-block-time` is optional and it defaults to 3000ms if not set
after setting `--dev`.
* to start OmniNode with manual seal it is enough to pass just `--dev`.
* `--dev-block-time` can still be used to start a node with manual seal,
but it will not set it up as `--dev` does (it will not set a bunch of
flags which are enabled by default when `--dev` is set: e.g. `--tmp`,
`--alice` and `--force-authoring`.

Closes: #6537

## Integration

Relevant for node/runtime developers that use OmniNode lib, including
`polkadot-omni-node` binary, although the recommended way for runtime
development is to use `chopsticks`.

## Review Notes

* Decided to focus only on OmniNode & templates docs in relation to it,
and leave the `parachain-template-node` as is (meaning `--dev` isn't
usable and testing a runtime with the `parachain-template-node` still
needs a relay chain here). I am doing this because I think we want
either way to phase out `parachain-template-node` and adding manual seal
support for it is wasted effort. We might add support though if the
demand is for `parachain-template-node`.
* Decided to not infer the block time based on AURA config yet because
there is still the option of setting a specific block time by using
`--dev-block-time`. Also, would want first to align & merge on runtime
metadata checks we added in Omni Node here:
https://github.com/paritytech/polkadot-sdk/pull/6450 before starting to
infer AURA config slot duration via the same way.

- [x] update the docs to mention `--dev` now.
- [x] mention about chopsticks in the context of runtime development

---------

Signed-off-by: Iulian Barbu <iulian.barbu@parity.io>
Co-authored-by: Michal Kucharczyk <1728078+michalkucharczyk@users.noreply.github.com>