whitepaper: Add ROI section.

[bwplotka]

Suggestion from our final 1.0 community review period:

Improving your Observability - "Return on Investment"* - Implementing fine-grained Observability can be extremely expensive, especially if done naively. Usually, we recommend starting simple and iterate with more signals, more granularity and data sizes. Let's discuss those suggestions in a new section. Let's also enumerate how we should iterate and what criteria have to be met to proceed with observability expansion.

[mhausenblas]

Maybe Return on Investment Driven Observability could be useful?

[kumarankit999]

Can I work on this, if no one is working on this issue

[alolita]

@kumarankit999 What specifically do you want to work on?

You can submit a draft on this section as a PR and that can be considered when reviewed.