Add Exporting FAQ/Troubleshooting guide

**Your Godot version:**
3.2.1
**Issue description:**
I would like to propose an exporting FAQ/troubleshooting guide.

There is a wealth of information already available in the documentation, but the relevance of it to exporting is not always immediately obvious. A user running into errors might not think to look in the filesystems page, for example, although it contains critical information to the exporting process (in this case, the fact that res:// becomes read only in a build, where it is read/write in the editor).

A quick search through the community Discord revealed many similar questions from the past several months. I propose the following export-related details be specifically clarified on the Exporting page:

1. .json, .txt, and .csv files are blocked from export by default and need to be manually added back in with an export filter (are there any other common file types I'm missing there?)
2. res:// becomes read-only in a build. So not only will calls to write to a file in res:// not work, but simply opening a file with read/write access will crash/softlock/otherwise break the project, even if that write access is not used.

I believe having these and other future details easily accessible in the documentation, either within an existing page or as a page of its own, will decrease confusion and reduce questions in the Discord and other support channels.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/getting_started/step_by_step/exporting.html#
https://docs.godotengine.org/en/stable/getting_started/step_by_step/filesystem.html#resource-path