Updated documentation related to payment (Stripe)

[annstriganova]

1. Task's name:

Update Payment endpoint documentation

2. Developer's full name:

Anna Striganova

3. Changes:

• updated PlantUML diagrams
• updated OpenAPI docs
• updated START.md
• removed unused code

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Files	Coverage Δ	Complexity Δ
...edlatte/email/sender/PaymentEmailConfirmation.java	33.33% <ø> (ø)	1.00 <0.00> (ø)
.../api/scenario/SessionCompletedScenarioHandler.java	9.09% <ø> (ø)	1.00 <0.00> (ø)
...ar/icedlatte/payment/endpoint/PaymentEndpoint.java	7.14% <ø> (+0.89%)	1.00 <0.00> (ø)