add links to other tutorials to tpil4

Stream: lean4

[Bulhwi Cha]

Current documentation in introduction.md:

These aspects of Lean are explored in a companion tutorial to this one,
[Programming in Lean 4](TBD), though computational aspects of the system will
make an appearance here.

Suggestion:

These aspects of Lean are explored in two companion tutorials, Functional Programming in Lean and Metaprogramming in Lean 4, though computational aspects of the system will make an appearance here.

GitHub PR: https://github.com/leanprover/theorem_proving_in_lean4/pull/65