Merge pull request #2640 from pguibert6WIND/doc_ldp

doc: add Ldp user documentation

diff --combined doc/Makefile.am
index 62cb3c2edb452e7e859fe6eafef2d863c7ca4a08,631571e235c13406107217702beb172ba9a04ada..19aab63ea3609a8db574cc3b43c00414d089ae6f
--- 1/doc/Makefile.am
--- 2/doc/Makefile.am
+++ b/doc/Makefile.am
@@@ -183,7 -183,6 +183,7 @@@ EXTRA_DIST = frr-sphinx.mk 
        developer/ldpd-basic-test-setup.md \
        developer/library.rst \
        developer/Makefile.in \
 +      developer/maintainer-release-build.rst \
        developer/memtypes.rst \
        developer/modules.rst \
        developer/next-hop-tracking.rst \
@@@ -193,6 -192,7 +193,7 @@@
        developer/workflow.rst \
        developer/zebra.rst \
        user/babeld.rst \
+       user/ldpd.rst \
        user/basic.rst \
        user/bgp.rst \
        user/bugs.rst \