feat: add cloudwatch client docs for Swift and Kotlin

[thisisabhash]

Description of changes:

Adds public API documentation for the new standalone CloudWatch client (AmplifyCloudWatchClient) on Swift and Kotlin.
New pages:

• Frontend client page — [platform]/frontend/logging/cloudwatch: installation, client
  initialization, configuration options, registering as an Amplify logging sink, emitting
  messages directly, flushing, enable/disable, setting the user identifier, updating logging
  constraints, observing events, escape hatch, error handling, and CloudWatch Logs service limits.
• Backend setup page — [platform]/build-a-backend/add-aws-services/logging/cloudwatch:
  CDK snippet to create a log group + IAM policy, and a raw IAM policy fallback.

Both pages are registered under the Logging category in src/directory/directory.mjs.

The client is experimental, so each page leads with a warning that these public APIs are
subject to change and are not meant for production code, along with the platform-specific
opt-in (@_spi(AmplifyExperimental) import on Swift, @OptIn(ExperimentalAmplifyApi::class)
on Android). API surface (types, signatures, options, and error types) was taken from the
merged code in amplify-swift (AmplifyCloudWatchClient) and amplify-android (aws-cloudwatch).

Related GitHub issue #, if available:

N/A

Instructions

Which product(s) are affected by this PR (if applicable)?

• amplify-libraries

Which platform(s) are affected by this PR (if applicable)?

• Swift
• Android

Checks

• Does this PR conform to the styleguide?
• Does this PR include filetypes other than markdown or images? — No (.mdx and .mjs nav config only).
• Are any files being deleted with this PR? — No.
• Are all links in MDX files using the MDX link syntax rather than HTML link syntax?

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