Update the pallet guide in `sdk-docs` (#4735)

After using this tutorial in PBA, there was a few areas to improve it.
Moreover, I have:

- Improve `your_first_pallet`, link it in README, improve the parent
`guide` section.
- Updated the templates page, in light of recent efforts related to in
https://github.com/paritytech/polkadot-sdk/issues/3155
- Added small ref docs about metadata, completed the one about native
runtime, added one about host functions.
- Remove a lot of unfinished stuff from sdk-docs
- update diagram for `Hooks`