build: distribute the frost, chilldkg and iceberg documentation

README.md gained three links when the modules landed -- to
src/modules/frost/frost.md, src/modules/chilldkg/chilldkg.md and
doc/iceberg.md -- and none of the three files was added to EXTRA_DIST,
so make dist produced a tarball whose README pointed at files that were
not in it.

doc/iceberg.md is the clearest case: every other file under doc/ is
listed, so it was simply missed. The two module documents follow the
existing pattern for module-local documentation, where
src/modules/surjection/surjection.md and
src/modules/whitelist/whitelist.md are equally absent and equally linked
from README.md. That pattern is inherited and is left for a separate
change; adding the two this fork introduced at least makes the fork's own
additions self-consistent.

Verified with a from-scratch autogen.sh, configure and make dist in a
clean copy of the tree: the tarball now carries doc/ellswift.md,
doc/musig.md, doc/iceberg.md, src/modules/frost/frost.md and
src/modules/chilldkg/chilldkg.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Makefile.am

@@ -318,7 +318,8 @@ maintainer-clean-local: clean-testvectors
 ### Additional files to distribute
 EXTRA_DIST = autogen.sh CHANGELOG.md SECURITY.md
 EXTRA_DIST += doc/release-process.md doc/safegcd_implementation.md
-EXTRA_DIST += doc/ellswift.md doc/musig.md
+EXTRA_DIST += doc/ellswift.md doc/musig.md doc/iceberg.md
+EXTRA_DIST += src/modules/frost/frost.md src/modules/chilldkg/chilldkg.md
 EXTRA_DIST += examples/EXAMPLES_COPYING
 EXTRA_DIST += sage/gen_exhaustive_groups.sage
 EXTRA_DIST += sage/gen_split_lambda_constants.sage