bpf, docs: Split the comparism to classic BPF from instruction-set.rst

Split the introductory that explain eBPF vs classic BPF and how it maps
to hardware from the instruction set specification into a standalone
document.  This duplicates a little bit of information but gives us a
useful reference for the eBPF instrution set that is not encumbered by
classic BPF.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-3-hch@lst.de

Documentation/bpf/index.rst

@@ -21,6 +21,7 @@ that goes into great technical depth about the BPF Architecture.
    helpers
    programs
    maps
+   classic_vs_extended.rst
    bpf_licensing
    test_debug
    other