doc: remove old doc

[weichweich]

Remove old doc. https://dev.kilt.io should be our only documentation source. We never update the stuff here anyways..

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• This PR does not introduce new custom types
  • If not, I have opened a companion PR with the type changes in the KILT types-definitions repository
• I have left the code in a better state
• I have documented the changes (where applicable)

[wischli]

LGTM