docs: fix package-doc example, typos, and stale links

Correct the pkg.go.dev package example so it compiles and behaves as described: use the lowercase receiver b, compare Count() == 1 so the "Intersection works." branch is reachable, and use the full import path.

Document the exported EachSet iterator, which was the only exported symbol missing a doc comment, and fix comment typos in Len, Clear, UnionCardinality, and SetBitsetFrom. Align the package overview with the README on automatic shrinking.

Point the README Test and Go Report Card badges at bits-and-blooms instead of the old willf org, and fix a typo in the security policy.

Include the required -pkg=bitset flag in the pextgen regeneration instructions and regenerate pext.gen.go so its header matches.