build: fix and simplify doc/index.rst generation

* Fix usage of `echo "...\n..."` (without `-e`), generating a
broken output:
```
   lightningd-config <lightningd-config.5.md>
   lightningd\nlightning-autocleaninvoice.7.md <lightningd.8.md\nlightning-autocleaninvoice.7.md>
```

* Use `$(SORT)` instead of bare `sort` to avoid locale-dependent sort.

* Strip the trailing .[0-9].md without invoking sed.

* Sort all man pages as one block.

Changelog-None

doc/Makefile

@@ -97,6 +97,6 @@ doc-clean:
 	$(RM) doc/deployable-lightning.{aux,bbl,blg,dvi,log,out,tex}
 
 doc/index.rst:
-	(grep -v '^   lightning.*\.[0-9]\.md>$$' $@; for m in $$(cd doc && echo "$$(ls lightningd*.[0-9].md | sort)\n$$(ls lightning-*.[0-9].md | sort)"); do echo "   $$(echo $$m | sed 's/.[0-9].md//') <$$m>"; done) > $@.tmp.$$$$ && mv $@.tmp.$$$$ $@
+	(grep -v '^   lightning.*\.[0-9]\.md>$$' $@; for m in $$(cd doc && ls lightningd*.[0-9].md lightning-*.[0-9].md); do echo "   $${m%.[0-9].md} <$$m>"; done |$(SORT)) > $@.tmp.$$$$ && mv $@.tmp.$$$$ $@
 
 .PHONY: doc/index.rst

doc/index.rst

@@ -27,8 +27,6 @@ c-lightning Documentation
    :maxdepth: 1
    :caption: Manpages
 
-   lightningd <lightningd.8.md>
-   lightningd-config <lightningd-config.5.md>
    lightning-autocleaninvoice <lightning-autocleaninvoice.7.md>
    lightning-check <lightning-check.7.md>
    lightning-checkmessage <lightning-checkmessage.7.md>
@@ -69,3 +67,5 @@ c-lightning Documentation
    lightning-waitinvoice <lightning-waitinvoice.7.md>
    lightning-waitsendpay <lightning-waitsendpay.7.md>
    lightning-withdraw <lightning-withdraw.7.md>
+   lightningd <lightningd.8.md>
+   lightningd-config <lightningd-config.5.md>