Closes https://github.com/paritytech/polkadot-sdk-docs/issues/70

WIP PR for an overview of how to develop tokens in FRAME. 

- [x] Tokens in Substrate Ref Doc
  - High-level overview of the token-related logic in FRAME
- Improve docs with better explanation of how holds, freezes, ed, free
balance, etc, all work
- [x] Update `pallet_balances` docs
  - Clearly mark what is deprecated (currency)
- [x] Write fungible trait docs
- [x] Evaluate and if required update `pallet_assets`, `pallet_uniques`,
`pallet_nfts` docs
- [x] Absorb https://github.com/paritytech/polkadot-sdk/pull/2683/


- [x] Audit individual trait method docs, and improve if possible

Feel free to suggest additional TODOs for this PR in the comments

---------

Co-authored-by: Bill Laboon <[email protected]>
Co-authored-by: Francisco Aguirre <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>

Fixes https://github.com/paritytech/polkadot-sdk/issues/3887.

Currently, all protocols use the same metric name for
`mpsc-notification-to-protocol` this is bad because we can't actually
tell which protocol might cause problems.

This patch proposes we derive the name of the metric from the protocol
name, so that we have separate metrics for each protocol and properly
detect which one is having problem processing its messages.

---------

Signed-off-by: Alexandru Gheorghe <[email protected]>

Removes transient code introduced to clean up offchain database after
`im-online` pallet removal.

Should be merged after #2290 has been enacted.

Somehow https://github.com/paritytech/polkadot-sdk/pull/3795

 was merged
but tests are failing now on master. I suspect that CI is not even
running these tests anymore which is a big issue.

---------

Signed-off-by: Andrei Sandu <[email protected]>

PR adds variables validation and app credentials for pushing into
gh-pages

cc https://github.com/paritytech/ci_cd/issues/934