Remove refs to "Registered types section" in how-to build/parse files

[johnmhoran]

Remove the references to "Registered types section" -- inherited from the OG .rst -- in:

• https://github.com/package-url/purl-spec/blob/main/docs/how-to-build.md
• https://github.com/package-url/purl-spec/blob/main/docs/how-to-parse.md