[RF-DOCS][ci skip] Update Active Record Encryption Guide

[bhumi1102]

Motivation

The purpose of this PR is to update the Active Record Encryption Guide. The Active Record Encryption guide is currently labeled work-in-progress (WIP).

Detail

• Update introduction and basic setup

• Maybe rename "Supported Types" to "Serialized Attributes"?
  "Supported types" could apply to a lot of things (encryption types, column types?) which might be confusing.
• Features is a bit of a grab bag of items.
  • Maybe move some of those sections to the "Basic Usage" section? For example "Supported Types", "Ignoring Case" and "Compression" describe options to the encrypts method and seem pretty common. Maybe this should also apply to "Action Text" and "Fixtures".
• Maybe create a new section, something like "Migrating Existing Data" that includes "Support for Unencrypted Data" and "Support for Previous Encryption Schemes"?
• I'm not sure "API" deserves its own section. "API" also isn't very descriptive. Some sections could probably be integrated under Basic Usage or maybe removed?
• The "Configuration Options" section can probably be removed because these options should already be described in https://edgeguides.rubyonrails.org/configuring.html
  Left the configuration options in for now. The Configuration guide has a couple but not all 14 of them. Maybe it makes sense to move them all there...
• The "Encryption Contexts" section should probably be a top section instead.
• Maybe describe with_encryption_context first in/before "Built-in Encryption Contexts"? This would make describing without_encryption a bit clearer as it passes the NullEncryptor context to with_encryption_context.
• The subsections of "Encryption Context" should probably be mimic "Key Management": "Built-in Encryption Contexts", "Custom Encryption Contexts", "Attribute Encryption Contexts".