feat(docs): Improve the toolchain setup page (#2272)

Split the toolchain setup into separate docker and native pages
and improve instructions to better refer to Zephyr docs in certain steps.
Also refactor to improve consistency and add virtualenv instructions.

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Co-authored-by: KemoNine <mcrosson@kemonine.info>
Co-authored-by: Cem Aksoylar <caksoylar@users.noreply.github.com>

docs/blog/2020-08-12-zmk-sotf-1.md

@@ -19,7 +19,7 @@ There's been lots of various activity in ZMK land!
 - Tons of [documentation](/docs) work.
 - Refactoring ([#73](https://github.com/zmkfirmware/zmk/pull/73), [#74](https://github.com/zmkfirmware/zmk/pull/74)) of [keymaps](/docs/features/keymaps) to make them simpler for users.
 - Mod-Tap Behavior (docs coming!) is much improved ([#69](https://github.com/zmkfirmware/zmk/pull/69)) and usable now.
-- An initial [`setup.sh`](http://localhost:3000/docs/user-setup#user-config-setup-script) script was created, allowing users to quickly bootstrap a "user config" setup and push it to GitHub, where GitHub Actions will build the firmware for you.
+- An initial [`setup.sh`](/docs/user-setup#user-config-setup-script) script was created, allowing users to quickly bootstrap a "user config" setup and push it to GitHub, where GitHub Actions will build the firmware for you.
 - Corne shield ([#80](https://github.com/zmkfirmware/zmk/pull/80)) shield definition was added.
 - Initial [encoder](/docs/features/encoders) support ([#61](https://github.com/zmkfirmware/zmk/pull/61)) was added.