Vertex mapping for all functions that return a subgraph

[!WARNING]
This is still a draft, the bounty has no legal value yet.

As recently pointed out on Discourse, some functions such as steiner_stree return a subgraph without remembering which vertices from the original graph were selected. The original graph might have had vertices 1:N and the subgraph 1:n, but no mapping 1:n -> 1:N is provided.
This contrasts with functions like induced_subgraph which do return this mapping.
It would be nice to write down an exhaustive list of every subgraph-returning function in Graphs.jl, and make sure that each of them has a method/alternative that allows mapping back into the original graph.

The bounty completion requires API implementation, documentation (potentially also including good See also entries and a dedicated page), and testing.

Required skills: Some familiarity with the average julia package-dev process.

Reviewer: @gdalle or members of the JuliaGraphs community

Duration: 1 month (except potential review overhead)

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Start by reviewing steiner_tree and induced_subgraph, then inventory the other Graphs.jl functions that return subgraphs and determine how each can preserve a mapping to the original graph. Done means the relevant APIs, documentation and tests cover the complete list of subgraph-returning functions and their vertex mappings.