Show more state borders at low zoom levels (#979)

* Change NE 50m by NE 10m state provinces dataset

* update documentation

Fixes #977 

This PR implements the change discussed at #977 to replace Natural Earth 10m states and provinces by the 50m version, since the 10m only covers state borders for US, Canada, Brazil, and Australia.

A few remarks:

* This change only affects zooms 1 to 4, not making any change in borders based in the `OSM Borders`  source. 
* The `min_zoom<=7` shows the first level regions (level 4 in OSM) as you can check for example for France, Italy, or Spain.
* I don't think it's necessary to simplify the geometries, it would mean adding a new materialized view, or a slower tile generation process.
* This change does not need any change in the styles, even someone could argue it could be interesting to add the country code in this layer as a new field, allowing style editors to filter features by country, but I would prefer to leave that change for another PR.

layers/boundary/boundary.sql

@@ -225,7 +225,7 @@ FROM ne_110m_admin_0_boundary_lines_land
     );
 
 -- etldoc: ne_50m_admin_0_boundary_lines_land  -> boundary_z1
--- etldoc: ne_50m_admin_1_states_provinces_lines -> boundary_z1
+-- etldoc: ne_10m_admin_1_states_provinces_lines -> boundary_z1
 -- etldoc: osm_border_disp_linestring_gen11 -> boundary_z1
 CREATE OR REPLACE VIEW boundary_z1 AS
 (
@@ -243,7 +243,8 @@ SELECT geometry,
        NULL AS disputed_name,
        NULL AS claimed_by,
        FALSE AS maritime
-FROM ne_50m_admin_1_states_provinces_lines
+FROM ne_10m_admin_1_states_provinces_lines
+WHERE min_zoom <= 7
 UNION ALL
 SELECT geometry,
        admin_level,
@@ -256,7 +257,7 @@ FROM osm_border_disp_linestring_gen11
 
 
 -- etldoc: ne_50m_admin_0_boundary_lines_land -> boundary_z3
--- etldoc: ne_50m_admin_1_states_provinces_lines -> boundary_z3
+-- etldoc: ne_10m_admin_1_states_provinces_lines -> boundary_z3
 -- etldoc: osm_border_disp_linestring_gen11 -> boundary_z3
 CREATE OR REPLACE VIEW boundary_z3 AS
 (
@@ -274,7 +275,8 @@ SELECT geometry,
        NULL AS disputed_name,
        NULL AS claimed_by,
        FALSE AS maritime
-FROM ne_50m_admin_1_states_provinces_lines
+FROM ne_10m_admin_1_states_provinces_lines
+WHERE min_zoom <= 7
 UNION ALL
 SELECT geometry,
        admin_level,
@@ -308,7 +310,7 @@ SELECT geometry,
        NULL AS claimed_by,
        FALSE AS maritime
 FROM ne_10m_admin_1_states_provinces_lines
-WHERE min_zoom <= 5
+WHERE min_zoom <= 7
 UNION ALL
 SELECT geometry,
        admin_level,