Revisit reference docs for TransactionExtension

[georgepisaltu]

Follow up on #2280

The introduction of TransactionExtension changed the semantics of extensions and extrinsics, which now don't need to provide any signature types. This means the current entries in docs/sdk/src/reference_docs/transaction_extensions.rs and docs/sdk/src/reference_docs/extrinsic_encoding.rs are outdated.

Using the information in the PR description of #2280 and its PrDoc, we need to update the reference docs.

[georgepisaltu]

@jsdw acknowledging your effort on #3234, would you like to contribute here?

[georgepisaltu]

Also mentioning the FRAME examples which need updating.

[jsdw]

I'd love to write the ref docs (and still have writing the metadata ref docs on my todo list); I have quite a lot to get through at the moment but I'll try to find time for this in the next few weeks! If somebody else beats me to it then that's totally fine too.