Release automation script

Things that are not currently happening in this workflow:
- No GPG-signature on the Git tag
- No *.asc signature file for the tarball (or for any other release assets)
- No GPG-signed Debian and other OBS packages

To-do:
- remove the corresponding entries from
  https://github.com/fish-shell/fish-shell/wiki/Release-checklist
  and link to this workflow.
- Maybe add some testing (for the Linux packages)?.
- Let's hope that this doesn't cause security issues.

Usage:
1. run "build_tools/release.sh $version"; this will create and push
   a tag, which kicks off .github/workflows/release.yml
2. wait for the draft release to be created at
   https://github.com/fish-shell/fish-shell/releases/tags/$version
3. publish the draft (manually, for now). This should unblock the
   last part of the workflow (website updates).

Closes #10449

Incremental usage example:

	version=4.0.3
	repository_owner=fish-shell
	remote=origin
	cd ../fish-shell-secondary-worktree
	git tag -d $version ||:
	git push $remote :$version ||:
	git reset --hard origin/Integration_$version
	for d in .github build_tools; do {
		rm -rf $d
		cp -r ../fish-shell/$d .
		git add $d
	} done
	git commit -m 'Backport CI/CD'
	echo "See https://github.com/$repository_owner/fish-shell/actions"
	echo "See the draft release at https://github.com/$repository_owner/fish-shell/releases/$version"
	../fish-shell/build_tools/release.sh $version $repository_owner $remote

doc_src/conf.py

@@ -45,8 +45,8 @@ def issue_role(name, rawtext, text, lineno, inliner, options=None, content=None)
     return [link], []
 
 
-def do_not_use_fish_indent_for_man(app):
-    if app.builder.name == "man":
+def remove_fish_indent_lexer(app):
+    if app.builder.name in ("man", "markdown"):
         del lexers["fish-docs-samples"]
 
 
@@ -65,7 +65,7 @@ def setup(app):
     app.add_config_value("issue_url", default=None, rebuild="html")
     app.add_role("issue", issue_role)
 
-    app.connect("builder-inited", do_not_use_fish_indent_for_man)
+    app.connect("builder-inited", remove_fish_indent_lexer)
 
 
 # The default language to assume