cmake: speed up curldown processing, enable by default

- cmake: enable `BUILD_DOCS` by default (this controls converting and
  installing `.3` files from `.md` sources)

- cmake: speed up generating `.3` files by using a single command per
  directory, instead of a single command per file. This reduces external
  commands by about a thousand. (There remains some CMake logic kicking
  in resulting in 500 -one per file- external `-E touch_nocreate` calls.)

- cd2nroff: add ability to process multiple input files.

- cd2nroff: add `-k` option to use the source filename to form the
  output filename. (instead of the default in-file `Title:` line.)

Follow-up to 3f08d80b2244524646ce86915c585509ac54fb4c
Follow-up to ea0b575dab86a3c44dd1d547dc500276266aa382 #12753
Follow-up to eefcc1bda4bccd800f5a56a0fe17a2f44a96e88b #12730

Closes #12762

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