Add architecture docs

[nayihz]

For beginners, It's a little hard to get involved in blixt. Maybe we need more documents to tell users/developers about how we design blixt, how we use ebpf and so on.

[shaneutt]

We definitely need to do this, but as the design is rapidly changing during these v0.x.x releases, it will be a little bit of a moving target. As such this should be on hold until v0.8.0 (which is when we expect things to settle down more).

/priority important-longterm
/hold