docs(framework) Update Flower Architecture documentation

[WilliamLindskog]

Issue

Very little text documentation on Flower Architecture.

Description

Adding small description about different underlying engines.

Related issues/PRs

N/A

Proposal

Write a bit more text about it.

Explanation

Describing the engines in Flower architecture.

Checklist

• [ x] Implement proposed change
• [ x] Write tests
• [ x] Update documentation
• [ ] Make CI checks pass
• [ x] Ping maintainers on Slack (channel #contributions)

Any other comments?

[WilliamLindskog]

@tanertopal @danieljanes , just updated header. Lmk if there is anything about the content that I can change.

[WilliamLindskog]

just updated title, should be correct format.