Fix 404: Add conformance.md and changes-from-previous-iso.md to the website build

[bact]

• Fix 404 of "Conformance" chapter
  • https://spdx.github.io/spdx-spec/v3.0.1-draft/conformance/ <-- currently 404 not found
  • conformance.md was in nav of mkdocs.yml, but also in draft_docs, made it excluded from the website build and thus 404.
  • Fixed by removed it from draft_docs
• Fix 404 of "Changes from the previous standard" chapter
  • Corrected the file name to changes-from-previous-iso.md

Also raise the validation level of not_found to warn, so the next time an issue like this will be detected during PR submission.

[bact]

There's one warning in validation, from the missing omg-history.md

WARNING - A reference to 'omg-history.md' is included in the 'nav' configuration, which is not found in the documentation files.

PR #1053 has be applied before this PR

[goneall]

LGTM