Influxdb3 runtime thread alloc

[jstirnaman]

Closes #6403

Adds documentation building on the mentioned Slack discussion:

• Add runtime architecture documentation covering thread pools, resource allocation, and concurrency model

[jstirnaman]

Feedback:

After skimming the "performance tuning" page: I noticed that some of the example commands have flags in the wrong places. 3ent is sometimes peculiar about the ordering of flags and commands. For example, --num-io-threads flag only works before the serve command. I suggest breaking the page into a few parts: first part is general principles: must monitor cpu usage (per core), memory, io (network); then what few settings I might use to adjust cpu, threads, and memory; then as a final section the encyclopedic appendix, probably as a separate page. I found it hard to see what information might be useful to me and as an operator while I should understand the full performance tuning docs, usually I'm looking for something more succinct: "my ingesters are only using 2 cores --> increase io_threads in my ingesters --> done". Perhaps the "common performance issues" section should be first in the page. Less is more, and specifics are golden. For example, the docs suggest Increase WAL flush frequency but doesn't say how to do that (it does mention --wal-flush-interval but far in the page from where "Increase WAL flush frequency" is mentioned).

--io-runtime-type [env: INFLUXDB3_IO_RUNTIME_TYPE=] [default: multi-thread]: should be marked as a dev flag. Don't recommend using anything but multi-thread which is the default. The current-thread option will be gone in 3.5 3ent (running with just one thread is probably not a good idea generally anyway)