Tactic chapter/FAQ: add creation of a fresh unique name

Proposal:

Add the following q&a to the FAQ:

Q: How do I create a fresh unique name?

A: Use Lean.Core.mkFreshUserName <name-basis>.

This creates a new (unused) inaccessible name based on name-basis.

elab " faq_fresh_hyp_name " : tactic =>
  Lean.Elab.Tactic.withMainContext do
    -- create fresh name based on name `h`
    let h := Lean.mkIdent (← Lean.Core.mkFreshUserName `h)
    -- create new hypothesis with this fresh name
    Lean.Elab.Tactic.evalTactic (← `(tactic| have $h : 1 + 1 = 2 := by simp))
    -- use hypothesis
    Lean.Elab.Tactic.evalTactic (← `(tactic| rewrite [$h:ident]))
    -- remove hypothesis
    Lean.Elab.Tactic.evalTactic (← `(tactic| clear $h))

example : 1 + 1 = 2 := by
  faq_fresh_hyp_name
  rfl

Locate the tactic chapter/FAQ and read the surrounding entries to match its formatting and organization. Add the proposed question, answer, and Lean example, then verify that the FAQ explains fresh unique names and that the example demonstrates creating, using, and clearing the hypothesis.