Aliases to old pages used within OSCAL model documentation

[nikitawootten-nist]

The following URLs are used within OSCAL model documentation (in the metaschemas):

• https://pages.nist.gov/OSCAL/concepts/layer/implementation/component-definition/
• https://pages.nist.gov/OSCAL/concepts/layer/overview/#modeling-approach
• https://pages.nist.gov/OSCAL/concepts/uri-use/#absolute-uri
• https://pages.nist.gov/OSCAL/concepts/uri-use/#linking-to-another-oscal-object
• https://pages.nist.gov/OSCAL/concepts/uri-use/#relative-reference
• https://pages.nist.gov/OSCAL/concepts/uri-use/#use-as-a-naming-system-identifier

These URLs contribute to 554 total dead links within the OSCAL Reference

[aj-stein-nist]

Since this is an important page and a low effort PR you just published, I will bring this into sprint. Thanks for bringing this to our attention, @nikitawootten-nist!