[text analytics] Update readme

[iscai-msft]

• get rid of --pre in pip install section
• follow .net for header and splitting of AAD auth and AzureKeyCredential auth
• remove some examples from key concepts (generally this section doesn't really have examples, but looking at it, have decided to keep all of the examples)
• Update samples links to absolute (this is potentially causing a problem on the docs.microsfot.com rendering of samples)
• Rename "Client" section in Key Concepts to "TextAnalyticsClient"

[iscai-msft]

cc @czubair