docs: Update outdated design and API documentation

[ayushiahjolia]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Issue Link, if available

#184

Description

Update all docs under docs/.

• Fix step signatures from Supplier to Function<StepContext, T>
• Add missing operations to design.md API surface (callback, waitForCallback, runInChildContext, parallel, waitForCondition)
• Rewrite parallel.md from outdated design plan to actual API (ParallelDurableFuture, branch(), ParallelResult)
• Fix waitForCondition initialState from method param to config
• Fix renamed types (CompletionReason -> ConcurrencyCompletionStatus, NOT_STARTED -> SKIPPED, WaitForConditionException -> WaitForConditionFailedException, InvokeTimedoutException -> InvokeTimedOutException)
• Add missing DurableConfig options (pollingStrategy, checkpointDelay)
• Complete exception hierarchy with all 20 exception classes
• Add missing packages/classes to design.md package structure
• Fix broken links and remove non-existent InvokeConfig.timeout()
• Update ADR-003 status from Review to Accepted
• Removed spec/ folder, it was outdated

Demo/Screenshots

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Checklist

• I have filled out every section of the PR template
• I have thoroughly tested this change

Testing

Unit Tests

Have unit tests been written for these changes? N/A

Integration Tests

Have integration tests been written for these changes? N/A

Examples

Has a new example been added for the change? (if applicable) N/A