docs: add examples for cachix usage

docs/cachix.md

@@ -0,0 +1,46 @@
+# Cachix usage
+
+URL: https://pub-solar.cachix.org
+
+Example to build and push a custom package of a host in this flake (e.g. after creating an overlay):
+
+```
+nix build --json -f . '.#nixosConfigurations.nachtigall.pkgs.keycloak^*' \
+  | jq -r '.[].outputs | to_entries[].value' \
+  | cachix push pub-solar
+```
+
+Example to build and push a package in the  `nixpkgs` repo:
+
+```
+cd nixpkgs
+nix build --json -f . 'pkgs.lix^*' \
+  | jq -r '.[].outputs | to_entries[].value' \
+  | cachix push pub-solar
+```
+
+
+Checking if a package has been correctly pushed to the cache:
+
+```
+❯ nix build --json '/nix/store/f76xi83z4xk9sn6pbh38rh97yvqhb5m0-noto-fonts-color-emoji-png-2.042.drv^*' | jq -r '.[].outputs | to_entries[].value'  | cachix push pub-solar
+Pushing 1 paths (0 are already present) using zstd to cache pub-solar ⏳
+
+✓ /nix/store/xpgpi84765dxqja3gd5pldj49xx2v0xl-noto-fonts-color-emoji-png-2.042 (10.30 MiB)
+
+All done.
+
+❯ curl -I https://pub-solar.cachix.org/xpgpi84765dxqja3gd5pldj49xx2v0xl.narinfo
+HTTP/2 200
+date: Mon, 26 Aug 2024 09:31:10 GMT
+content-type: text/x-nix-narinfo
+traceparent: 00-b99db37cc9c2581b8d226cdf81e54507-794fc49193659c03-01
+tracestate:
+cache-control: public, max-age=14400
+last-modified: Mon, 26 Aug 2024 09:31:10 GMT
+cf-cache-status: EXPIRED
+report-to: {"endpoints":[{"url":"https:\/\/a.nel.cloudflare.com\/report\/v4?s=A67KGsCIsYjoFdvndxJ0rkmb7BZ5ztIpm8WUJKAiUPRVWvbYeXU9gU27P7zryiUtArbwrLzHhhMija0yyXk0kwNa3suz8gNzKK6z1CX1FWDZiiP07rnq7zAg8nZbSBiEU%2FZrU9nSrR6mhuL9ihbmW1Hf"}],"group":"cf-nel","max_age":604800}
+nel: {"success_fraction":0,"report_to":"cf-nel","max_age":604800}
+server: cloudflare
+cf-ray: 8b92ceab0d19c80e-DUS
+```