docs: add `TURBO_CONCURRENCY` to options overview page #6312

Job	Run time
Find path changes	18s
Turborepo Test Summary	2s
Rust lints	0s
Turborepo rust check	0s
@turbo/types codegen check	0s
`non-monorepo` example	0s
`kitchen-sink` example	0s
Turborepo Rust testing on ${{ matrix.os.name }}	0s
`basic` example	0s
`with-svelte` example	0s
`with-tailwind` example	0s
Turborepo Integration	0s
Cleanup / cleanup	5s
	25s

Provide feedback