Update troubleshooting.md

Author: carlopi

docs/stable/extensions/troubleshooting.md

@@ -6,5 +6,23 @@ redirect_from:
 title: Troubleshooting of Extensions
 ---
 
-This page is a placeholder troubleshooting extension installs.
-For the time being, please refer to the [Installing Extensions page]({% link docs/stable/extensions/installing_extensions.md %}).
+You might be visiting this page directed via a DuckDB error message, similar to:
+```sql
+INSTALL non_existing;
+```
+```
+HTTP Error:
+Failed to download extension "non_existing" at URL "http://extensions.duckdb.org/v1.4.0/osx_arm64/non_existing.duckdb_extension.gz" (HTTP 404)
+
+Candidate extensions: "inet", "encodings", "core_functions", "sqlite_scanner", "postgres_scanner"
+For more info, visit https://duckdb.org/docs/stable/extensions/troubleshooting?version=v1.4.0&platform=osx_arm64&extension=non_existing
+```
+
+There are multiple scenarios for which an extensions might not be available in a given extension repository at a given time:
+* extension have not been uploaded yet, here some delay after a given release date might be expected. Consider checking the issues at duckdb/duckdb or duckdb/community-extensions, or creating one yourself.
+* extension is available, but in a different repository, try for example `INSTALL <name> FROM core;` or `INSTALL <name> FROM community;` or `INSTALL <name> FROM core_nightly;` (check https://duckdb.org/docs/stable/extensions/installing_extensions#extension-repositories) 
+* networking issues, so extension exists at the endpoint but it's not reachable from your local DuckDB. Here you can try visiting the given URL via a browser directly pasting the link from the error message in the search bar.
+
+If you are on a development version of DuckDB, that is any version for which `PRAGMA version` returns a library_version not starting with a `v`, then extensions might not be available anymore on the default extension repository.
+
+Consider raising an issue on duckdb/duckdb if in doubt.