Remove structured-documentation-generator

[jorritn]

The structured-documentation folder was supposed to be the single source of truth for many S2 documentation (e.g. on docs.s2standard.org but also in the S2 libraries) but the actual single source of truth is the S2 standard itself. So we have a different place to generate the TOMLs in the structured-documentation folder from. Also, the sturctured-documentation-generator was only supposed to be used once.

• Remove the structured documentation folder in this repository
• Update README, add a note that the TOMLs are managed externally.