mkdocs: Fix ref not found warnings

[bact]

(A rework of #926 to fix unresolvable rebase/merge conflicts)

Fix ref/doc not found warnings during spec doc generation.

Currently, mkdocs serve and mkdocs build emit these warnings:

WARNING -  A reference to 'model/Software/Properties/isDirectory.md' is included in the 'nav' configuration, which is not
           found in the documentation files.
WARNING -  A reference to 'model/Dataset/Classes/Dataset.md' is included in the 'nav' configuration, which is not found
           in the documentation files.
WARNING -  A reference to 'model/Dataset/Properties/sensitivePersonalInformation.md' is included in the 'nav'
           configuration, which is not found in the documentation files.
WARNING -  A reference to 'model/AI/Properties/sensitivePersonalInformation.md' is included in the 'nav' configuration,
           which is not found in the documentation files.
WARNING -  Doc file 'conformance.md' contains a link 'diffs-from-previous-editions.md', but the target is not found among
           documentation files.
WARNING -  Doc file 'conformance.md' contains a link 'SPDX-Lite.md', but the target is not found among documentation
           files.

The first 4 warnings are responsible to the four erroneous None class/properties displaying in the left pane of https://spdx.github.io/spdx-spec/v3.0/ . They are all old property names. New property names are already included in the nav. Fixed by removing all 4 outdated references:

• model/Software/Properties/isDirectory.md is now fileKind
  • Change isDirectory to fileKind enumeration spdx-3-model#666
• model/Dataset/Classes/Dataset.md is now DatasetPackage
  • Dataset: Rename Dataset class to DatasetPackage and update properties desc. spdx-3-model#671
• model/Dataset/Properties/sensitivePersonalInformation.md is now hasSensitivePersonalInformation
• model/AI/Properties/sensitivePersonalInformation.md is now useSensitivePersonalInformation
  • AI/Dataset: Rename sensitivePersonalInformation to make them unique spdx-3-model#656

The remaining 2 warnings are fixed by add directory name to the targets in conformance.md:

• diffs-from-previous-editions.md and SPDX-Lite.md are in annexes/ directory
• Note that conformance.md is not included yet in the 3.0 as the content is need to be updated for the version. So fixing these 2 warnings or not is not effecting what the public will see in the spec doc - yet, until we include the Conformance chapter to the spec.

[bact]

@goneall @kestewart this one is to replace the conflicted PR #926

[goneall]

LGTM

[goneall]

Since the previous PR was approved, I'll go ahead and merge this before another conflict

[kestewart]

Thanks Gary.