docs: update location of TESTGROUND_HOME to xdg directory specification

See testground/testground#1544 and https://github.com/adrg/xdg

Author: dougEfresh

concepts-and-architecture/builders.md

@@ -63,7 +63,7 @@ None of these options are required and need only be edited if the defaults do no
 
 ## Examples
 
-Single build for a single test for the example/output plan using the exec:go builder. This command will produce a binary which you can find in `~/testground/` on Linux and macOS systems.
+Single build for a single test for the example/output plan using the exec:go builder. This command will produce a binary which you can find in `~/.config/testground/` on Linux or `~/Library/Application Support/testground` on macOS systems.
 
 ```bash
 $ testground build single --plan=example --builder=exec:go --wait

getting-started.md

@@ -63,7 +63,7 @@ Configure `$TESTGROUND_HOME` and copy the example `network` test plan into the `
 
 ```bash
 # assuming you already started your Testground daemon (as instructed above)
-# there should be a `testground` directory in your home folder, i.e. `~/testground`
+# there should be a `testground` directory in your home folder, i.e. `~/.config/testground` or ~/Library/Application Support/testground
 #
 # from your testground/testground Git checkout, run:
 $ testground plan import --from ./plans/network

writing-test-plans/quickstart.md

@@ -14,10 +14,10 @@ Go through the [Getting started](../getting-started.md) page and follow the inst
 
 ### 2. Create a test plan
 
-Testground stores plans in the `$TESTGROUND_HOME` directory. This directory is created for you in your `home` directory, but if you prefer to store plans in another location, you can adjust the location using the environment variable.
+Testground stores plans in the `$TESTGROUND_HOME` directory. This directory is created for you in `~/.config/testground` (linux) or ` ~/Library/Application Support/testground` (mac), but if you prefer to store plans in another location, you can adjust the location using the environment variable.
 
 ```bash
-# OPTIONAL - adjust TESTGROUND_HOME, default is ~/testground
+# OPTIONAL - adjust TESTGROUND_HOME, default is ~/.config/testground or ~/Library/Application Support/testground on mac
 $ export TESTGROUND_HOME=/path/to/testground/home
 
 # create your first test plan