"--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>

--- a/doc/manual/local.mk
+++ b/doc/manual/local.mk
@@ -138,11 +138,6 @@
 	$(trace-gen) $(dummy-env) NIX_PATH=nix/corepkgs=corepkgs $(bindir)/nix __dump-builtins > $@.tmp
 	@mv $@.tmp $@
 
-# Generate the HTML manual.
-.PHONY: manual-html
-manual-html: $(docdir)/manual/index.html
-install: $(docdir)/manual/index.html
-
 # Generate 'nix' manpages.
 install: $(mandir)/man1/nix3-manpages
 man: doc/manual/generated/man1/nix3-manpages
@@ -167,23 +162,4 @@
 	done
 	@touch $@
 
-$(docdir)/manual/index.html: $(MANUAL_SRCS) $(d)/book.toml $(d)/anchors.jq $(d)/custom.css $(d)/src/SUMMARY.md $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md $(d)/src/command-ref/conf-file.md $(d)/src/language/builtins.md
-	$(trace-gen) \
-		tmp="$$(mktemp -d)"; \
-		cp -r doc/manual "$$tmp"; \
-		find "$$tmp" -name '*.md' | while read -r file; do \
-			$(call process-includes,$$file,$$file); \
-		done; \
-		find "$$tmp" -name '*.md' | while read -r file; do \
-			docroot="$$(realpath --relative-to="$$(dirname "$$file")" $$tmp/manual/src)"; \
-			sed -i "s,@docroot@,$$docroot,g" "$$file"; \
-		done; \
-		set -euo pipefail; \
-		RUST_LOG=warn mdbook build "$$tmp/manual" -d $(DESTDIR)$(docdir)/manual.tmp 2>&1 \
-			| { grep -Fv "because fragment resolution isn't implemented" || :; }; \
-		rm -rf "$$tmp/manual"
-	@rm -rf $(DESTDIR)$(docdir)/manual
-	@mv $(DESTDIR)$(docdir)/manual.tmp/html $(DESTDIR)$(docdir)/manual
-	@rm -rf $(DESTDIR)$(docdir)/manual.tmp
-
 endif
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</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=3485984a090edd5b1721a69a389a2e44077354e7&amp;showmsg=1'>root</a>/<a href='/guix/log/tests?id=3485984a090edd5b1721a69a389a2e44077354e7&amp;showmsg=1'>tests</a>/<a href='/guix/log/tests/status.scm?id=3485984a090edd5b1721a69a389a2e44077354e7&amp;showmsg=1'>status.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/status.scm?id=3485984a090edd5b1721a69a389a2e44077354e7'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2022-06-26 23:19:14 +0200'>2022-06-26</span></td><td class='logsubject'><a href='/guix/commit/tests/status.scm?id=f99f00fc814a3e1a3e1cedb5059c896e3303677c'>status: Relay "updating substitutes" messages.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Until now, those messages would be accumulated and displayed all at
once, when a '\n' was finally emitted by 'guix substitute'.  In the
meantime, clients would remain silent.

* guix/status.scm (bytevector-index): Change 'number' parameter to
'numbers' and adjust accordingly.
(build-event-output-port): Pass both #\newline and #\return to
'bytevector-index'.
* tests/status.scm ("build-output-port, daemon messages with LF"): New
test.


</td></tr>
<tr class='logheader'><td><span title='2022-06-26 23:19:13 +0200'>2022-06-26</span></td><td class='logsubject'><a href='/guix/commit/tests/status.scm?id=c31605b58228dbd10c819311a17341a22c9e5118'>status: Change tests from SRFI-11 to SRFI-71.</a></td><td>Ludovic Courtès</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* tests/status.scm: Use SRFI-71 'let' instead of SRFI-11 'let-values'.


</td></tr>