Add attestation performance explainer to docs

We should add https://hackmd.io/@dapplion/lodestar_attestation_summary to our docs to make it more discoverable / visible. We could also create a dedicated page about effectiveness and provide instructions on how to debug sub optimal performance, e.g. what metrics to look at in which order, what logs to look at, important timings etc.

Similar to https://lighthouse-blog.sigmaprime.io/attestation-analysis.html