"--enable-doc-gen" configure flag will enable HTML document generation that requires mdbook.
We can't simply disable it because we need manpages.

Author: Zhu Zihao <all_but_last@163.com>

diff --git a/doc/manual/local.mk b/doc/manual/local.mk
index e43d9f2fb..c323d1847 100644
--- a/doc/manual/local.mk
+++ b/doc/manual/local.mk
@@ -69,8 +69,6 @@ $(d)/builtins.json: $(bindir)/nix
 	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(bindir)/nix __dump-builtins > $@.tmp
 	@mv $@.tmp $@
 
-# Generate the HTML manual.
-install: $(docdir)/manual/index.html
 
 # Generate 'nix' manpages.
 install: $(mandir)/man1/nix3-manpages
@@ -94,7 +92,5 @@ doc/manual/generated/man1/nix3-manpages: $(d)/src/command-ref/new-cli
 	done
 	@touch $@
 
-$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/command-ref/conf-file.md $(d)/src/expressions/builtins.md $(call rwildcard, $(d)/src, *.md)
-	$(trace-gen) RUST_LOG=warn mdbook build doc/manual -d $(DESTDIR)$(docdir)/manual
 
 endif
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<div class='path'>path: <a href='/guix/log/?id=f9ed5788fda9288301550c641820d422e9ad1602'>root</a>/<a href='/guix/log/tests?id=f9ed5788fda9288301550c641820d422e9ad1602'>tests</a>/<a href='/guix/log/tests/store-roots.scm?id=f9ed5788fda9288301550c641820d422e9ad1602'>store-roots.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/tests/store-roots.scm?id=f9ed5788fda9288301550c641820d422e9ad1602&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-08-18 16:10:08 +0200'>2023-08-18</span></td><td><a href='/guix/commit/tests/store-roots.scm?id=4b1fdd44ed87dc07d23cc9313a8b6b14d0646923'>tests: store-roots: Remove bogus test.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2023-06-18 23:36:25 +0200'>2023-06-18</span></td><td><a href='/guix/commit/tests/store-roots.scm?id=1261ce15233cbf7e24de9959df86a23b46314a28'>store: Tolerate non-existent GC root directories.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2019-04-10 17:09:47 +0200'>2019-04-10</span></td><td><a href='/guix/commit/tests/store-roots.scm?id=72eda0624be89ed18302fd7d7f22976071ab020c'>Add (guix store roots).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>