docs: add customization hints for secondary importance

Removing the download links for now as the tile importance
is still too experimental.

docs/customize/Importance.md

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+## Importance
+
+Search requests can yield multiple results which match equally well with
+the original query. In such case Nominatim needs to order the results
+according to a different criterion: importance. This is a measure for how
+likely it is that a user will search for a given place. This section explains
+the sources Nominatim uses for computing importance of a place and how to
+customize them.
+
+### How importance is computed
+
+The main value for importance is derived from page ranking values for Wikipedia
+pages for a place. For places that do not have their own
+Wikipedia page, a formula is used that derives a static importance from the
+places [search rank](../customize/Ranking#search-rank).
+
+In a second step, a secondary importance value is added which is meant to
+represent how well-known the general area is where the place is located. It
+functions as a tie-breaker between places with very similar primary
+importance values.
+
+nominatim.org has preprocessed importance tables for the
+[primary Wikipedia rankings](https://nominatim.org/data/wikimedia-importance.sql.gz)
+and for a secondary importance based on the number of tile views on openstreetmap.org.
+
+### Customizing secondary importance
+
+The secondary importance is implemented as a simple
+[Postgis raster](https://postgis.net/docs/raster.html) table, where Nominatim
+looks up the value for the coordinates of the centroid of a place. You can
+provide your own secondary importance raster in form of an SQL file named
+`secondary_importance.sql.gz` in your project directory.
+
+The SQL file needs to drop and (re)create a table `secondary_importance` which
+must as a minimum contain a column `rast` of type `raster`. The raster must
+be in EPSG:4326 and contain 16bit unsigned ints
+(`raster_constraint_pixel_types(rast) = '{16BUI}'). Any other columns in the
+table will be ignored. You must furthermore create an index as follows:
+
+```
+CREATE INDEX ON secondary_importance USING gist(ST_ConvexHull(gist))
+```
+
+The following raster2pgsql command will create a table that conforms to
+the requirements:
+
+```
+raster2pgsql -I -C -Y -d -t 128x128 input.tiff public.secondary_importance
+```