This PR introduces the new crate `developer_hub` into the polkadot-sdk
repo. The vision for the developer-hub crate is detailed in [this
document](https://docs.google.com/document/d/1XLLkFNE8v8HLvZpI2rzsa8N2IN1FcKntc8q-Sc4xBAk/edit?usp=sharing).

<img width="1128" alt="Screenshot 2023-11-02 at 10 45 48"
src="https://github.com/paritytech/polkadot-sdk/assets/5588131/1e12b60f-fef5-42c4-8503-a3ba234077c3">


Other than adding the new crate, it also does the following: 

* Remove the `substrate` crate, as there is now a unique umbrella crate
for multiple things in `developer_hub::polkadot_sdk`.
* (backport candidate) A minor change to `frame-support` macros that
allows `T::RuntimeOrigin` to also be acceptable as the origin type.
* (backport candidate) A minor change to `frame-system` that allows us
to deposit events at genesis because now the real genesis config is
generated via wasm, and we can safely assume `cfg!(feature = "std")`
means only testing. related to #62.
* (backport candidate) Introduces a small `read_events_for_pallet` to
`frame_system` for easier event reading in tests.
* From https://github.com/paritytech/polkadot-sdk-docs/issues/31, it
takes action on improving the `pallet::call` docs.
* From https://github.com/paritytech/polkadot-sdk-docs/issues/31, it
takes action on improving the `UncheckedExtrinsic` docs.

## Way Forward

First, a version of this is deployed temporarily
[here](https://blog.kianenigma.nl/polkadot-sdk/developer_hub/index.html).
I will keep it up to date on a daily basis.

### This Pull Request

I see two ways forward: 

1. We acknowledge that everything in `developer-hub` is going to be WIP,
and merge this asap. We should not yet use links to this crate anywhere.
2. We make this be the feature branch, make PRs against this, and either
gradually backport it, or only merge to master once it is done.

I am personally in favor of option 1. If we stick to option 2, we need a
better way to deploy a staging version of this to gh-pages.

### Issue Tracking

The main issues related to the future of `developer_hub` are: 

- https://github.com/paritytech/polkadot-sdk-docs/issues/31
- https://github.com/paritytech/polkadot-sdk-docs/issues/4
- https://github.com/paritytech/polkadot-sdk-docs/issues/26 
- https://github.com/paritytech/polkadot-sdk-docs/issues/32
- https://github.com/paritytech/polkadot-sdk-docs/issues/36


### After This Pull Request

- [ ] create a redirect for
https://paritytech.github.io/polkadot-sdk/master/substrate/
- [x] analytics 
- [ ] link checker
- [ ] the matter of publishing, and how all of these relative links for
when we do, that is still an open question. There is section on this in
the landing page.
- [ ] updated https://paritytech.github.io/

---------

Co-authored-by: Liam Aharon <[email protected]>
Co-authored-by: Juan Girini <[email protected]>
Co-authored-by: bader y <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: James Wilson <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>