docs: explain how to build cdn js file

[ForestJohnson]

I was trying to figure out how to re-build the same file that is served from CDNs, and I got confused and tripped over myself on the way. through my own mistakes, I had actually convinced myself that the build script in this repo did not produce the file that the CDN was serving. Nothing in the docs talked about it, so I just assumed it was some "special thing that exists somewhere else."

I am adding this to the docs now so that the next person reading through will be re-assured and hopefully find the right file.

[kurkle]

Not sure if this would be valuable or just confuse people, because I'm not sure it is common to try to reproduce something in cdn. What about NPM etc.

[ForestJohnson]

Ok, then how about this? instead of

file you would get from a CDN will be located at ./dist/chart.umd.js

it could say

if you want to include the Chart.js javascript file that you just built directly on an HTML page instead of importing it as a module, use dist/chart.umd.js

I kind of doubt that I'm the only chartjs documentation reader who doesn't know what umd means and doesn't know that I should look for "jsdelivr": "./dist/chart.umd.js", in package.json to learn that

"chart.umd.js is the one you can just plop onto the page in a script tag"