Configure spotless for markdown formatting/validation

[breedx-splk]

In other repos we have spotless configured for validating and reformatting markdown files. This helps to prevent thrash between commits, especially around small formatting changes and whitespace.

We should add that to our spotless configuration in this repo as well.

[marcschaeferger]

@breedx-splk I think the Spotless configuration for Markdown is already in place. See:

• https://github.com/marcschaeferger/opentelemetry-android/blob/main/buildSrc/src/main/kotlin/otel.spotless-conventions.gradle.kts
• For comparison: https://github.com/open-telemetry/opentelemetry-java/blob/main/buildSrc/src/main/kotlin/otel.spotless-conventions.gradle.kts

When I compare them, the Markdown part looks the same. Is there anything else you expect to be included?

One thing we could add: a .markdownlint.yaml like in opentelemetry-go (https://github.com/open-telemetry/opentelemetry-go/blob/main/.markdownlint.yaml) and wire those rules into our Spotless checks. Example .markdownlint.yaml we could start from:

default: true
MD013:
  line_length: 120
MD033: false
MD041: false

If you agree, I can prepare a PR that:

• Adds a .markdownlint.yaml to the repo,
• Ensures Spotless picks up those rules (and updates the existing buildSrc file if needed),
• Runs formatting/validation in CI/Github Actions.

Thoughts?