Include common html code

There is no include support with HTML.

Some HTML editors support there own way of doing this with special commands placed inside comments and replacing the content upon saving. But this will not help you with your current task because you have to edit all files at least once and must always use the specific editor later.

But you might use a text editor that provides search and replace for multiple files (all opened or all matching within a directory). With such an editor search for the existing menu entry code just before or after the new entry and replace with the old code followed / prefixed with the new one. Try first with a single file opened in the editor and check the result before applying to all files because most editors does not support undo for operations on multiple files.

[EDIT]
An editor that provides such operations is Notepad++.

You mentioned SSI (Server Side Includes). These are handled (as indicated by the name) by the web server which will load the file content (or variable contents or the output of an script) when delivering the page content. It must be enabled by web server configuration and the HTML file must be executable (by file system bit or using a specific extension). It will not work for local files loaded with the browser, requires to be supported by the web server, and requires access to the web server configuration.

So you might use it if you are in a position to change the web server configuration.
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