doc lint: make "lint-docs" non-.PHONY

doc lint: make "lint-docs" non-.PHONY

Speed up the "lint-docs" target by making it non-.PHONY. Similar to my
c234e8a0ecf (Makefile: make the "sparse" target non-.PHONY,
2021-09-23). We'll now create empty files corresponding to a
dependency graph for each of these lint scripts.

This speeds things up a bit[1], and makes the output correspond to any
in-tree changes we have:

    $ touch git-add.txt; make lint-docs; make lint-docs
        GEN cmd-list.made
        GEN doc.dep
        LINT GITLINK git-add.txt
        LINT MAN END git-add.txt
        LINT MAN SEC git-add.txt
    make: Nothing to be done for 'lint-docs'.

As with the "sparse" target changes this has a hard dependency on the
use of ".DELETE_ON_ERROR" in the Makefile, added here in
db10fc6c09f (doc: simplify Makefile using .DELETE_ON_ERROR,
2021-05-21). This method also depends on the output for us emitting
any errors on STDERR (fixed in a preceding commit), as well us these
scripts exiting with non-zero on any errors (which they were already
doing).

1.
$ git show HEAD~:Documentation/Makefile >Makefile.old
$ hyperfine --warmup 2 -L f ",.old" 'make -j1 -f Makefile{f} lint-docs'
Benchmark #1: make -j1 -f Makefile lint-docs
  Time (mean ± σ):      60.8 ms ±   1.4 ms    [User: 58.7 ms, System: 2.5 ms]
  Range (min … max):    58.9 ms …  64.0 ms    48 runs

Benchmark #2: make -j1 -f Makefile.old lint-docs
  Time (mean ± σ):      84.0 ms ±   1.5 ms    [User: 78.6 ms, System: 5.7 ms]
  Range (min … max):    81.8 ms …  87.8 ms    35 runs

Summary
  'make -j1 -f Makefile lint-docs' ran
    1.38 ± 0.04 times faster than 'make -j1 -f Makefile.old lint-docs'

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>