Improve Documentation

[jzheaux]

Each section of Spring Security's reference manual should be reviewed in consideration of the following structure:

• Start with "what to do"
  • show code samples that cover 90% of applications
  • show testing what was added
  • link to relevant Spring Security Sample commit diffs
  • link to the next relevant documentation step(s)
• Follow with "how it works"
  • use diagrams to explain workflows
  • include tradeoffs and security principles
  • show tabular information
• Follow with 10% usage

and the following principles:

• Linked "what to dos" should make progressive sense together
• Use headers for contextual linking
• Favor the latest recommendations
• Explain deprecated recommendations relative to the benefits of the latest ones
• Not have outdated recommendations
• Use XML/Java/Kotlin tabs
• Link to other Spring projects; Framework, Data, Boot, MVC, etc.

Here are the sections that are planned so far:

• Revisit Logout Documentation #13062
• Revisit Session Management docs #12601
• Revisit Authorization Documentation #13088
• Revisit CSRF Documentation #13089
• Revisit OAuth 2.0 Client Documentation #13090
• OAuth 2.0 Resource Server
• Revisit SAML 2.0 Service Provider Documentation #14944
• Authentication
• Response Headers
• Firewall
• Remember Me