Clarify spec for subpath

[johnmhoran]

johnmhoran: 2025-01-20 18:39: Just started to flesh out the relevant issues, PRs and comments before converting this to an issue.

Relevant issues and PRs:

• Clarifications, in particular about version encoding #35
• docs(types): oci type repositry_url must be parcent encoding #150
• test: unencoded subpath cannot contain . or .. #368

We also need to

• review the relevant language etc. in PURL-SPECIFICATION.rst
• apply RFC 2119/8174 but replace the use of any "not"/"NOT" with affirmative language

[jkowalleck]

will work on this.

i'll give this a bit of time, and wait until #361 got merged,
so i can use the newly added structures from there.