Organize the repo numbering and formatting

[1ucian0]

• [x] Create an index https://github.com/Qiskit/rfcs/pull/40/
  • include what are RFC that are still valid (deprecate those related to Aqua, for example)
• Change formats:
  • [x] 0001 to md https://github.com/Qiskit/rfcs/pull/33
  • [x] 0003 to md https://github.com/Qiskit/rfcs/pull/37
  • [x] 0004 formatting issues https://github.com/Qiskit/rfcs/pull/35
  • [x] Aqua Circuit Interoperability Proposal.ipynb to md, as 0005 https://github.com/Qiskit/rfcs/pull/36
• [x] Fill the gaps: https://github.com/Qiskit/rfcs/pull/38/
  • 0008-0011 missing
  • 00013-0024 missing
  • 0026-0029 missing
  • [x] Numbering ####-repo-rename.md https://github.com/Qiskit/rfcs/pull/39

[1ucian0]

All tasks done. Issue done.