This adds a top-level document for the new `from` command, with a list of links of all the subcommands.
All the from-* subcommands keep their filename, but the content is updated to use the new subcommand syntax.
Needs matching update for to*
Ref #1709
fixes#969, admittedly without a --delimiter alias
moves from_structured_data.rs to from_delimited_data.rs to better
identify its scope and adds to_delimited_data.rs. Now csv and tsv both
use the same code, tsv passes in a fixed '\t' argument where csv passes
in the value of --separator
