[Documentation] Fix references to the memmap crate in documentation so that

[bzm3r]

(This is mostly a TODO for me, unless someone else would like to follow up on it.)

• Currently, there are places in the documentation that reference the memmap crate, for example here (where the link also is broken). Given that fst now uses memmap2, these references should be updated. Furthermore, note the RustSec advisory around memmap. Therefore, examples that use memmap should also be updated to use memmap2.

• Highlight whether or not OpBuilder provides the ability to efficiently merge/split existing Fsts versus simply rebuilding the Fst from scratch. Update: OpBuilder [does not "fuse"] existing, lexicographically disjoint FSTs together.

• In the subsection Quirks, provide a link to merge.rs