Fix links in terms and definitions

[bact]

• Add links to ISO vocabularies (both are free)
• Make IETF RFC URL format consistent with other instances in the spec (for example, as in references.md)
  • Use format: https://datatracker.ietf.org/doc/rfc<number>/, from https://github.com/spdx/spdx-3-model/blob/develop/docs/format.md#writing-style
• Make links clickable (currently they are not, see https://spdx.github.io/spdx-spec/v3.1-dev/terms-and-definitions/)
• Remove trailing spaces

[bact]

I slightly updated the ISO standard names with an em dash (—), to make it the same as in the References https://github.com/spdx/spdx-spec/blob/develop/docs/references.md

Ping @gregshue for review