docs(man): colitur-overlay(5), the overlay format in full

The overlay format was documented in three partial places -- a paragraph in
colitur(1), a block in --help, and the comments inside the shipped example --
none of which was a reference. Someone writing a diocesan calendar had to
read all three and infer the rest.

Section 5 because an overlay is a thing a user AUTHORS rather than a command
they run: it belongs beside fstab(5), not in man1.

Covers every directive and every field edit, the six required fields and the
two optional ones, all three date specifications including the signed
Easter_offset and the negative nth, three worked examples, and the caveats.
The subject field gets a note explaining that it is not decoration -- it
decides whether a feast displaces an occurring Sunday under RG 16(a).

Two things it says that the code says and the old prose did not. There is no
Set_status and no Set_date among the field edits, deliberately: changing an
entry's status or its date makes it a different celebration rather than an
edited one, so Replace is the right directive and the change stays visible in
`colitur check` output. And a local feast missing from output has usually
LOST its day under the general rubrics rather than failed to load -- the
engine applies precedence to overlay entries exactly as to universal ones,
which is the first thing an author hits and was written down nowhere.

Writing it caught a documentation bug before it shipped: a first draft listed
a Set_status edit that does not exist and omitted Remove_name that does. Every
documented edit is now cross-checked against overlay.mli.

Linked from colitur(1)'s SEE ALSO and its OVERLAYS section, and from --help.
The Makefile installs it into man5, removes it on uninstall, and the man and
doc targets lint both pages.

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