Create Building in Gitpod.md (iNavFlight#10004)

Gitpod offer by far the easierst and fastest method for building INAV targets. I wrote a small .md for those who want to have a step-by-step guide.

docs/development/Building in Gitpod.md

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+# Building in Gitpod
+
+Gitpod offers an online build environment for building INAV targets.
+## Setting up the environment
+
+1.  Go to https://gitpod.io/new
+1.  Paste `https://github.com/iNavFlight/inav/tree/master` into the field called "Select a repository"
+1.  Cick on the link that shows in the drop down and Gitpod will atomatically selects the adequate Editor and Browser
+1.  Leave the other fields as default and click "Continue". Your build environment will be created.
+1.  At the bottom of the page, you will see a command line. Type `make [TARGET]` and wait for the target to be built.
+1.  Your new target will be located in a folder called `bin`, which can be found at the top left of the page.
+
+NOTE: You can use this method to build your forks as well. Just paste in the link to your fork and follow the rest of the steps.
+
+You are done!