[Documentation:] Complete FormRecognizer README

[annelo-msft]

Updated Description:

• Check that links are working
• Add section header for Package Installation
• Add Troubleshooting section
• Add Next Steps Section
• Complete list of links at the top
• Reduce to one training sample
  --

Was:

Fill out remaining required sections.

Add concepts for:

• OCR Results/TextElements
• Confidence
• Bounding Box

Update receipt sample such that return result is called receiptCollection.

Call out GetFieldValues in a sample.

Compare/contrast LayoutClient and CustomModelClient

Call out where models are already trained and don't need to train a model to use them.

[annelo-msft]

Priority: initial draft before UX study

[maririos]

Sections to add:

• Install package with version
• Troubleshooting
• Next steps (links to samples)

[annelo-msft]

Update to description:

Call out GetFieldValues in a sample.
Compare/contrast LayoutClient and CustomModelClient

GetFieldValues is no longer there, but Fields.TryGetValues() should be illustrated in a sample.
LayoutClient and CustomModelClient are now both on FormRecognizerClient - instead of compare/contrast these clients, compare/contrast StartRecognizeContent with StartRecognizeCustomForm -- the latter adds a layer of semantic information over OCR.