Fixes #4842

Description

Allow explicit (install) stanzas to override the auto-detected odig doc
files (README*, LICENSE*, CHANGE*, HISTORY*).

Dune automatically installs those files found in the package directory into
the doc section of every package. Until now there was no way to replace
them: adding an (install) stanza that installs e.g.
(LICENSE.md.<package> as LICENSE.md) for a single package produced a
Multiple rules generated for .../doc/<package>/LICENSE.md error, so there
was no way to ship a different LICENSE file per opam package in a
multi-package project.

I now drop an auto-detected doc entry when an explicit user-written
install entry targets the same (section, dst). The explicit entry wins and
no duplicate rule is generated. Auto-detection is unchanged when there is no
explicit override.

Fixes #4842

Verification

The relevant test for this area is the existing odig workspace test:

./dune.exe runtest test/blackbox-tests/test-cases/odig-files-workspace.t

plus a full check with:

./dune.exe build @check @fmt @runtest

I could not run them here: this environment has no OCaml toolchain
(_boot/dune.exe is absent and neither dune nor ocaml is on PATH), so
no test could be run. The change is confined to stanzas_to_entries in
src/dune_rules/install_rules.ml and only filters Dune-sourced Doc
entries that collide with a User-sourced entry on (section, dst).