doc: don't carry upstream bootstrap.texi.

* docs/bootstrap.texi: Remove.
* README.md: Add reference to upstream project for latest docs,
and remove reference to local copy.
* src/src.mk (man_MANS): Generate mkrockspecs.1 in standard 'doc'
subdirectory.
(EXTRA_DIST, CLEANFILES): Update accordingly.
* specs/mkrockspecs_spec.yaml: Adjust 'docs' references
accordingly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>