Minor grammar, style, and clarity improvements in spec - sections Introduction to Data Entities

[elichad]

Lots of very minor changes from proofreading the first few sections of the spec. For example:

• fixing incorrect spelling/grammar
• consistently using "an RO-Crate" rather than "a RO-Crate"
• consistently italicising and capitalising keywords like RO-Crate Root
• adding missing links in references
• reverting the font size for most headings to Bootstrap defaults as some of them are currently too small and not distinct from the text (see for example https://www.researchobject.org/ro-crate/specification/1.2-DRAFT/data-entities.html#referencing-other-ro-crates)

Anything that needs more discussion or careful double-checking I have saved for a separate issue/PR so as not to hide important things in a sea of tiny changes :)