express.static(path, [options]) returns a function. So basically what your code is doing is :
router.get('/v1/secure-api-documentation',ensureAuthenticate,(req,res)=>{
express_static_function // this function further accepts arguments req, res, next
//there is no function call happening here, so this is basically useless
});
However, this is not what express.static is used for
What express.static does is, takes the request path and looks for a file with the same name in the folder you specified.
Basically, if a GET request comes to '/v1/secure-api-documentation', it will take the request path after '/v1/secure-api-documentation' and look for that inside api_docs folder.
Passing express.static to router.get() will call it for the very SPECIFIC path. This is important.
GET '/v1/secure-api-documentation/index.html' will fail. Because such a route is not handled.
What you need to do this is call express static for any path like '/v1/secure-api-documentation/*'.
For this you need to take the express app object, and write the following code:
//make sure to use the change the second argument of path.join based on the file where your express app object is in.
app.use('/v1/secure-api-documentation',express.static(path.join(__dirname,'../api-doc')));
This will now work for not only the index.html file but any js/css file inside api_docs that is requested.