docs: add v3 migration guide (#3290)

Adds a guide that shows how to upgrade from v2 to v3

---------

Co-authored-by: Cayman <[email protected]>

Author: achingbrain

.github/dictionary.txt

@@ -1,28 +1,31 @@
-setbit
+additionals
+backpressure
+blpop
+buildx
+Certicom
+chacha
+connmanager
+dialback
+dout
 getbit
-stopstr
-rlflx
-incrby
 hopr
-dout
-supercop
+incrby
+microtask
 nothrow
-buildx
-blpop
+peerStore
+reprovide
+reprovided
+reprovider
+reprovides
+reproviding
+rlflx
 rpush
-additionals
-SECG
-Certicom
 RSAES
+SECG
+setbit
+stopstr
+supercop
 unuse
-dialback
-chacha
-peerStore
+userland
 xxhandshake
 zerolen
-connmanager
-reprovide
-reprovider
-reproviding
-reprovides
-reprovided