Updates to Implementation Notes

[rosy1280]

• [x] Section 1: Put OAIS in references. Link to it.
• [x] Section 4: include conformance declaration as reserved file
• [x] Section 5: Change heading to Storage
• [x] Section 5: Change "the paths of physical stored files" to "stored file paths"
• [x] Section 5 Change "version by reference to the manifest entry, not the actual path on disk." to "version by referencing the manifest entry, not the actual path on disk."
• [x] Overall: Typos and spelling
• [x] Overall: Add basic file operation section before section 7
• [x] Section 9: link to the fixity block section of the spec
• [x] Overall: Consider linking to definitions
• [x] Overall: Add a section about Storage Root Hierarchy

[zimeon]

https://github.com/OCFL/spec/pull/161 does number 1 above and some typos

[neilsjefferies]

Can we link to specific sections in the spec?

[zimeon]

@neilsjefferies IMO yes, we can and should link to sections of the spec from the implementation notes