chore(docs): Adds dredd tests for calculate-amount API

[blossom2017]

Fixes #7005
#7014

Short description of what this resolves:

Adds documentation for calculate-amount API endpoint.

Changes proposed in this pull request:

• Adds documentation in orders.apib for calculate-amount endpoint

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

[codecov]

Codecov Report

Merging #7263 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #7263   +/-   ##
============================================
  Coverage        63.55%   63.55%           
============================================
  Files              259      259           
  Lines            13034    13034           
============================================
  Hits              8284     8284           
  Misses            4750     4750           

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[iamareebjamal]

Great work. Thanks!