Add more concept explainers

[dylandreimerink]

I have recently come across a lot of "concepts" (features / systems) that could use their own page. Such a page is good for a coherent explanation of a concept and also a location to link to from other pages. This issue is just a place to get this out of my head, will make into separate issues later:

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1. BPF trampolines (https://lwn.net/Articles/804937/)
2. BPF-to-BPF functions (https://github.com/torvalds/linux/commit/cc8b0b92a1699bc32f7fec71daa2bfc90de43a4d)
3. dynptrs (https://lwn.net/Articles/895885/)
4. kptrs (Kernel pointers) (https://lwn.net/Articles/907129/)
5. Custom datastructures (https://github.com/torvalds/linux/commit/958cf2e273f0929c66169e0788031310e8118722)
6. Open coded iterators (https://lore.kernel.org/all/[email protected]/#r)
7. Fentry hooks (https://github.com/torvalds/linux/commit/a319185be9f5ad13c2a296d448ac52ffe45d194c, https://github.com/torvalds/linux/commit/0dd061a6a115f25132989cbd591a25afb2dee086)
8. HID-BPF (https://docs.kernel.org/hid/hid-bpf.html)
9. Global data / variables