Doc 13590 mention FLE on encryption page

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Adds references to the SDK's field-level encryption to the encryption at rest docs.

• In the Encryption page, moved the standalone section of field-level encryption to the encryption at rest discussion.
• In the Native Encryption at Rest page, added note about field-level encryption as a possible alternative.

Note: couldn't manage to use a partial for both of these, even though the text is almost identical. Antora's fussy formatting strikes again.