[FR] add link to local copy of docs in `?`

[heliosdrm]

Context: https://discourse.julialang.org/t/documentation-feature-request/42711

This would possibly belong in basedocs.jl, right after:

**Welcome to Julia $(string(VERSION)).** The full manual is available at
    https://docs.julialang.org

A link to the index.html in the local copy of the docs would be useful.

[heliosdrm]

An alternative approach is proposed in #34278

[ViralBShah]

Newer discussion #44537.