Add documentation about Profile-Guided Optimization (PGO)

In https://github.com/GreptimeTeam/greptimedb/issues/1218 we found that PGO helps to optimize GreptimeDB performance. I think we need to add information about it in the documentation so GreptimeDB users/maintainers will be aware of this method to optimize their GreptimeDB installations further with their workloads.

Here are the examples of similar documentation in other projects:
* ClickHouse: https://clickhouse.com/docs/en/operations/optimizing-performance/profile-guided-optimization
* Databend: https://docs.databend.com/developer/community/contributor/pgo
* Vector: https://vector.dev/docs/administration/optimization/pgo/
* Nebula: https://docs.nebula-graph.io/3.5.0/8.service-tuning/enable_autofdo_for_nebulagraph/
* GCC: Official [docs](https://gcc.gnu.org/install/build.html), section "Building with profile feedback" (even AutoFDO build is supported)
* Clang:
- https://llvm.org/docs/HowToBuildWithPGO.html
- https://llvm.org/docs/AdvancedBuilds.html