Replace homepage in all TOML files (#5118)

A bit of a controversial move, but a good preparation for even further
reducing the traffic on outdated content of `substrate.io`. Current
status:

<img width="728" alt="Screenshot 2024-07-15 at 11 32 48"
src="https://github.com/user-attachments/assets/df33b164-0ce7-4ac4-bc97-a64485f12571">

Previously, I was in favor of changing the domain of the rust-docs to
something like `polkadot-sdk.parity.io` or similar, but I think the
current format is pretty standard and has a higher chance of staying put
over the course of time:

`<org-name>.github.io/<repo-name>` ->
`https://paritytech.github.io/polkadot-sdk/`

part of https://github.com/paritytech/eng-automation/issues/10