k_heap / sys_heap needs overview documentation

[andrewboie]

We've introduced new heap APIs to supersede k_mem_pool / sys_mem_pool, but have no overview documentation for it, just API doxygen.

I expect this file will need to be rewritten to reflect the new API set:

doc/reference/kernel/memory/pools.rst

[carlescufi]

@andrewboie and @andyross don't forget to update docs/releases/, section about API deprecation. Either as part of this or as part of #24358