Merge GSN branch into master (#1867)

* GSN basics guide (#1847)

* Fix linter error

* Initial draft for basic GSN guide

* Add CustomerAcquisitonCost link

* Review 'gas costs' phrasing

* Rename relay to relayer

* Apply suggestions from code review

Co-Authored-By: Santiago Palladino <spalladino@gmail.com>

* Improve transition to contract usage explanation

* Improve payment section phrasing

* Apply suggestions from code review

Co-Authored-By: Andrew B Coathup <28278242+abcoathup@users.noreply.github.com>

* Apply more review suggestions

* Add code

* Add unreleased warning

* GSN Docs Tweaks (#1860)

* Adjusts links

* Adjustments

* Update gsn.adoc

Fix typo

* Adjusts copy and links for GSN guide (#1865)

* Adjusts copy and links

* Formats links

* Adds website link

docs/modules/ROOT/nav.adoc

@@ -4,6 +4,7 @@
 .Basics
 * xref:access-control.adoc[Access Control]
 * xref:tokens.adoc[Tokens]
+* xref:gsn.adoc[Gas Station Network]
 * xref:crowdsales.adoc[Crowdsales]
 * xref:utilities.adoc[Utilities]