Update user-intro.md

[mjanowiecki]

Made small wordsmith changes and added section explaining the organization of this section. I'm not sure if we should move the Modeling content, Fedora, and Conceptual diagram sections to another page or frame them better? They seem a little random now, although excellent and necessary information

Purpose / why

Although this page is titled as an introduction to the user documentation, it didn't really seem to do that. So I tried adding an overview of what the reader could expect in the upcoming sections.

What changes were made?

• Changed title case to sentence case
• Changed references to Islandora 7 to Islandora Legacy (is this right?)
• Linked first mention of Fedora to its glossary entry
• Added section explaining organization of subsections
• Small word-choice changes

Verification

Interested Parties

• @Islandora/documentation

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?