Fix links to implementers' guide (#1865)

# Description

In a couple of cases, there were links pointing to the w3f github pages
domain. In other instances, there were links pointing to the old
polkadot repo's github pages. Both of these are now pointing to the
relevant links in
https://paritytech.github.io/polkadot-sdk/book/index.html.

These changes were made specifically because the w3f github pages
returns a 404, and while fixing the links, the old polkadot repo links
were touched up as well even if they do redirect properly.

This shouldn't affect anything as these are documentation link changes
only.