docs: add docs to contract interfaces

[TanvirDeol]

AXE-5852

Added documentation for interchain-token and axelar-operators

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.57%. Comparing base (4da911b) to head (e826c40).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ontracts/stellar-axelar-operators/src/interface.rs	0.00%	1 Missing ⚠️
contracts/stellar-example/src/contract.rs	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   95.60%   95.57%   -0.03%     
==========================================
  Files          67       68       +1     
  Lines        3638     3642       +4     
==========================================
+ Hits         3478     3481       +3     
- Misses        160      161       +1     

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[TanvirDeol]

@Milap Do you want me to apply changes like

• Remove “Returns: Ok(())”
• Put error codes in brackets [] to make them linkable in docs
• replace authenticate w/ authorize
• Must be called by [Self::owner] —> [Self::owner] must authorize.
• etc

to stellar-interchain-token-service?