Maintenance Page using irule

That isn't going to work. Your best bet is to add your HTML page as an iFile:

https://support.f5.com/kb/en-us/products/big-ip_ltm/manuals/product/ltm-implementations-11-5-0/27.html?sr=47873155

https://devcentral.f5.com/wiki/iRules.iFile.ashx

Then your iRule might look something like this:

when HTTP_REQUEST {
     Retrieve the file contents, send it in an HTTP 200 response and clear the temporary variable
    set ifileContent [ifile get "/Common/iFile-index.html"]
    HTTP::respond 200 content $ifileContent
    unset ifileContent
}