fix: Update broken polkadot.com documentation links

Replace deprecated polkadot.com URLs (returning 404 after site redesign)
with current equivalents:

- bags-list: staking-update-september-2021 -> wiki.polkadot.com/docs/learn-staking-advanced#bags-list
- fast-unstake: staking-update-february-2022 -> wiki.polkadot.com/learn/learn-nominator
- favicon: Use existing paritytech PNG logo (ICO was removed)
- papers: polkadot.com/papers -> github.com/polkadot-io/polkadot-white-paper
- approval checking blog: polkadot.com/blog -> wiki.polkadot.com/docs/learn-parachains-protocol

Author: nkpar

docs/sdk/src/lib.rs

@@ -25,7 +25,9 @@
 #![warn(rustdoc::private_intra_doc_links)]
 // Frame macros reference features which this crate does not have
 #![allow(unexpected_cfgs)]
-#![doc(html_favicon_url = "https://polkadot.com/favicon.ico")]
+#![doc(
+	html_favicon_url = "https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/docs/images/Polkadot_Logo_Horizontal_Pink_White.png"
+)]
 #![doc(
 	html_logo_url = "https://raw.githubusercontent.com/paritytech/polkadot-sdk/master/docs/images/Polkadot_Logo_Horizontal_Pink_White.png"
 )]

docs/sdk/src/polkadot_sdk/polkadot.rs

@@ -14,7 +14,7 @@
 //! 	- [Dashboard](https://polkadot-fellows.github.io/dashboard/)
 //! - [Polkadot Specs](http://spec.polkadot.network)
 //! - [The Polkadot Parachain Host Implementers' Guide](https://paritytech.github.io/polkadot-sdk/book/)
-//! - [Polkadot papers](https://polkadot.com/papers/)
+//! - [Polkadot papers](https://github.com/polkadot-io/polkadot-white-paper)
 //! - [JAM Graypaper](https://graypaper.com)
 //!
 //! ## Alternative Node Implementations 🌈
@@ -54,7 +54,7 @@
 //!   the validity of the block execution of multiple parachains using the same set of validators as
 //!   the Relay Chain. In practice, this means that the shards (parachains) share the same economic
 //!   security as the Relay Chain.
-//!   Learn about this process called [Approval Checking](https://polkadot.com/blog/polkadot-v1-0-sharding-and-economic-security#approval-checking-and-finality).
+//!   Learn about this process called [Approval Checking](https://wiki.polkadot.com/docs/learn-parachains-protocol).
 //! * A framework to build blockchains: In order to materialize the ecosystem of parachains, an easy
 //!   blockchain framework must exist. This is [Substrate](crate::polkadot_sdk::substrate),
 //!   [FRAME](crate::polkadot_sdk::frame_runtime) and [Cumulus](crate::polkadot_sdk::cumulus).

polkadot/roadmap/implementers-guide/src/node/backing/statement-distribution.md

@@ -22,7 +22,7 @@ in case of disputes. Any validator can be selected as the next relay-chain block
 advance for security reasons. As a result, all validators must have a up to date view of all possible parachain
 candidates + backing statements that could be placed on-chain in the next block.
 
-[This blog post](https://polkadot.com/blog/polkadot-v1-0-sharding-and-economic-security) puts it another way:
+[The wiki](https://wiki.polkadot.com/docs/learn-parachains-protocol) puts it another way:
 "Validators who aren't assigned to the parachain still listen for the attestations [statements] because whichever
 validator ends up being the author of the relay-chain block needs to bundle up attested parachain blocks for several
 parachains and place them into the relay-chain block."

substrate/frame/bags-list/src/lib.rs

@@ -54,7 +54,7 @@
 //! can be used.
 //!
 //! Additional reading, about how this pallet is used in the context of Polkadot's staking system:
-//! <https://polkadot.com/blog/staking-update-september-2021/#bags-list-in-depth>
+//! <https://wiki.polkadot.com/docs/learn-staking-advanced#bags-list>
 //!
 //! ## Examples
 //!

substrate/frame/fast-unstake/src/lib.rs

@@ -39,7 +39,7 @@
 //! *Being exposed with validator* from the point of view of the staking system means earning
 //! rewards with the validator, and also being at the risk of slashing with the validator. This is
 //! equivalent to the "Active Nominator" role explained in
-//! [here](https://polkadot.com/blog/staking-update-february-2022/).
+//! [here](https://wiki.polkadot.com/learn/learn-nominator/).
 //!
 //! Stakers who are certain about NOT being exposed can register themselves with
 //! [`Pallet::register_fast_unstake`]. This will chill, fully unbond the staker and place them