docs(common): improving documentation

- update esp_modem to use esp_docs
- migrated docs from github pages to docs.espressif.com

docs/esp_modem/generate_docs

@@ -0,0 +1,26 @@
+build-docs --target esp32 --language en
+
+cp -rf _build/en/esp32/html .
+rm -rf _build __pycache__
+
+# Modifes some version and target fields of index.html
+echo "<script type="text/javascript">
+window.onload =(function() {
+    var myAnchor = document.getElementById('version-select');
+    var mySpan = document.createElement('input');
+    mySpan.setAttribute('type', 'text');
+    mySpan.setAttribute('maxLength', '10');
+    mySpan.value = 'latest';
+    mySpan.setAttribute('disabled', true);
+    myAnchor.parentNode.replaceChild(mySpan, myAnchor);
+
+    var myAnchor = document.getElementById('target-select');
+    var mySpan = document.createElement('input');
+    mySpan.setAttribute('type', 'text');
+    mySpan.setAttribute('maxLength', '10');
+    mySpan.value = 'all targets';
+    mySpan.setAttribute('disabled', true);
+    myAnchor.parentNode.replaceChild(mySpan, myAnchor);
+
+})();
+</script>" >> html/index.html