Doc edits

[crandmck]

Changes in this pull request

Edits and reorganization of documentation.

I have a few open questions:

• Installation: What can we say about OS support? Mac only? I suspect it is meant to work on Windows and Linux, but Windows always requires extra work (i.e. using WSL2). If it hasn't been tested on other OSes then we should say so.
• Testing locally in the Git directory section: I don't understand what this means "If you just want to do local testing builds..." Does this mean local build/testing of the tool itself? If this is for building the tool from source, I would clarify a bit and move this section to a more appropriate location.
• This repo is referenced a number of times, but it's private and won't be visible to anyone outside the c2pa org: https://github.com/c2pa-org/testing-private/tree/main/cert-generation. Either don't mention it, replace with an alternative, or ask that it be made public ASAP.

[codecov-commenter]

Codecov Report

All modified lines are covered by tests :white_check_mark:

Comparison is base (be9092b) 68.46% compared to head (4975b7f) 68.46%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #85 +/- ## ======================================= Coverage 68.46% 68.46% ======================================= Files 2 2 Lines 501 501 ======================================= Hits 343 343 Misses 158 158 ```

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[crandmck]

Thanks for catching the typos, @scouten-adobe.

I have now installed a spell-checker extension to my VS Code to help avoid these kinds of embarrassing typos in the future!