Exclude video markup from text output

For the Plone Nuclia project, we are experimenting with various outputs for easier indexing. This PR excludes videos from only the text output from the `make text` command. It affects only the text builder.

Author: stevepiercy

docs/contributing/documentation/myst-reference.md

@@ -268,8 +268,10 @@ Note that the path must be absolute to support both submodules and the main docu
 Don't use file-relative paths.
 The above MyST markup renders as shown below.
 
+````{only} not text
 ```{video} /_static/user-manual/blocks/block-copy-cut.mp4
 ```
+````
 
 
 ### Diagrams and graphs with Graphviz