[OTHER] Update build description in README.md

### I have searched through the issues and didn't find my problem.

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### What would you like to share?

The build description in the README.md is incomplete and hard to read, and shouold be updated for completeness and readability's sake.

https://github.com/devicons/devicon/tree/develop#go-build-yourself

I've identified a few concrete things that should be improved, but I'm sure there are more, since I haven't gone through the whole build process successfully myself. So without further to do, here are a few of the things I think needs improvement:
1. `Install gulp (and gulp plugins)` should link to gulp and gulp plugins, and explain in short how to install it. Maybe we can even get rid of this if it can be automated through a build script?
2. This part is incomprehensible to me. Where should I extract the archive? What files do I keep/remove/replace? Do I keep parts of the zip contents, or just some?
> The next step is to click on Generate font and download the resulting archive. Extract the contents and you will find a [fonts](https://github.com/devicons/devicon/blob/develop/fonts) directory next to a style.css. Replace the contents of the fonts folder, rename style.css as [devicon.css](https://github.com/devicons/devicon/blob/develop/devicon.css) and follow the next step to build the final stylesheet.
4. There should be mention of the need for a server to host. I think this can be added to the README:
> The website will need access to your files via fetch so you'd need a server to host it.
For VSCode we recommend using the [Live Server](https://marketplace.visualstudio.com/items?itemName=ritwickdey.LiveServer) extension. You can then click on the `Go Live` in the bottom right corner of your VSCode status bar.
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After that open up http://127.0.0.1:5500/docs/ to view the website locally.

### Additional information

I'm labeling this issue as a discussion so if you have any suggestions to improving the readability of the `Build` section of the README, please feel free to write some comments below