[DOC] Add feature-flag evaluation section

Change in the documentation

Our docs currently have a feature-flagging page, which is a bit sparse. That's partially because we link to our concepts in openfeature.dev , but there's probably a bit more flagd-specific stuff that could be added here. I propose a new section:

"Evaluating flags with flagd"

• Discuss basics of evaluating flags with flagd, including identifying the flag, and supplying a context. Link to https://flagd.dev/architecture/#rpc-vs-in-process-evaluation and https://flagd.dev/reference/flag-definitions/. Do not go into too much detail here, we have lots of reference docs which go into fine details. Keep it high-level.
• Discuss errors and egde cases (flag-not-found, type-mismatches, and float/int flag conversion)
• Discuss events (how changing a flag definition in flagd will cause events to fire in implementing SDKs), link to https://openfeature.dev/docs/reference/concepts/events