adding constraints reference

Reviewed-by: gtema <artem.goncharov@gmail.com>
Co-authored-by: Hasko, Vladimir <vladimir.hasko@t-systems.com>
Co-committed-by: Hasko, Vladimir <vladimir.hasko@t-systems.com>

otc_metadata/templates/tox.ini.j2

@@ -24,7 +24,9 @@ commands = {posargs}
 # This env is invoked in the periodic pipeline and is therefore responsible to
 # build all relevant docs at once.
 [testenv:docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps =
+  -r{toxinidir}/doc/requirements.txt
+  -c https://raw.githubusercontent.com/opentelekomcloud-docs-swiss/docs-constraints/main/constraints.txt
 allowlist_externals =
   mkdir
   cp
@@ -40,7 +42,7 @@ commands =
 {%- endfor %}
 
 [testenv:docs-pdf]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 allowlist_externals =
   rm
   mkdir
@@ -59,7 +61,7 @@ commands =
 {% set loc = doc.rst_location | replace('/source', '') %}
 # HTML version
 [testenv:{{ doc.type }}]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 allowlist_externals =
   cp
   mkdir
@@ -78,7 +80,7 @@ commands =
 
 # Json version (for search)
 [testenv:json-{{ doc.type }}]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 allowlist_externals =
   cp
   mkdir
@@ -102,7 +104,7 @@ commands =
 {% if doc.pdf_name -%}
 # PDF version
 [testenv:{{ doc.type }}-pdf-docs]
-deps = -r{toxinidir}/doc/requirements.txt
+deps = {[testenv:docs]deps}
 allowlist_externals =
   rm
   mkdir