DOC-3396: Improve custom-toolbarbuttons and file-image-upload metadata (#4060)

Update description and keywords for custom-toolbarbuttons (addButton,
custom button), file-image-upload (images_upload_handler), and
upload-images pages to address Context7 benchmark Q5/Q6 (25/100 each).

Author: kemister85

modules/ROOT/pages/custom-toolbarbuttons.adoc

@@ -1,7 +1,7 @@
 = Toolbar buttons
 :navtitle: Toolbar buttons
-:description: Add a custom buttons to the {productname} {productmajorversion} toolbar.
-:keywords: toolbar, toolbarbuttons, buttons, toolbarbuttonsapi
+:description: Add custom toolbar buttons using editor.ui.registry.addButton and the setup callback. Create buttons that trigger commands or custom actions.
+:keywords: toolbar, toolbarbuttons, buttons, toolbarbuttonsapi, custom button, addButton, editor.ui.registry, setup callback, onAction
 
 == Use cases
 

modules/ROOT/pages/file-image-upload.adoc

@@ -1,6 +1,7 @@
 = Image and file options
 :navtitle: Images and files
-:description: These settings affect TinyMCE's image and file handling capabilities.
+:description: Configure image uploads to a server using images_upload_handler or images_upload_url. Handle file uploads with custom logic or a server endpoint.
+:keywords: images_upload_handler, images_upload_url, image upload, server, upload handler, file picker, images_file_types
 
 == Controlling or adjusting allowed image and file formats
 

modules/ROOT/pages/upload-images.adoc

@@ -1,8 +1,8 @@
 = Handling image uploads
 :navtitle: Image uploads
 :description_short: How to manage asynchronous image uploads.
-:description: How to manage asynchronous image uploads.
-:keywords: uploader, uploadImages, image, handler, asynchronous, async, paste_data_images, image, cors
+:description: Configure and manage asynchronous image uploads using images_upload_handler or images_upload_url. Upload images to a server endpoint with custom handling.
+:keywords: uploader, uploadImages, image, handler, asynchronous, async, paste_data_images, image, cors, images_upload_handler, images_upload_url, image upload, server
 
 {productname} uploads edited images with the image uploader. This complements {productname}'s image editing functionality.