"--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=5d22261db3d5099d745b9e8c8484545e546b319e'>root</a>/<a href='/guix/log/doc?id=5d22261db3d5099d745b9e8c8484545e546b319e'>doc</a>/<a href='/guix/log/doc/he-config-bare-bones.scm?id=5d22261db3d5099d745b9e8c8484545e546b319e'>he-config-bare-bones.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/doc/he-config-bare-bones.scm?id=5d22261db3d5099d745b9e8c8484545e546b319e&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-06-10 22:42:49 +0200'>2022-06-10</span></td><td><a href='/guix/commit/doc/he-config-bare-bones.scm?id=d859d8873ceb3c591210968cfd4c88e26fc89edc'>doc: Update example of a minimalistic home environment.</a><span class='msg-avail'>...</span></td><td>Andrew Tropin</td></tr>
<tr><td><span title='2021-10-09 19:24:18 +0300'>2021-10-09</span></td><td><a href='/guix/commit/doc/he-config-bare-bones.scm?id=0e8d2df0f1a4ab25c482e1c427c54e19903e62f3'>Move (gnu home-services) to (gnu home services).</a><span class='msg-avail'>...</span></td><td>Oleg Pykhalov</td></tr>
<tr><td><span title='2021-10-08 15:36:18 +0300'>2021-10-08</span></td><td><a href='/guix/commit/doc/he-config-bare-bones.scm?id=ba8ddb348045f81f061a1c7f51c0f7c2b0024e71'>gnu: Move (gnu home-services) to (gnu home services).</a><span class='msg-avail'>...</span></td><td>Oleg Pykhalov</td></tr>
<tr><td><span title='2021-09-09 20:26:51 +0300'>2021-09-09</span></td><td><a href='/guix/commit/doc/he-config-bare-bones.scm?id=aaffa43c1752e062934580f55616e0380963a1be'>doc: Add Guix Home documentation.</a><span class='msg-avail'>...</span></td><td>Andrew Tropin</td></tr>