doc: readme overhaul and azure sponsorship (#2826)

* new readme structure

* add adopters file

* add microsoft sponsorship

* add reference to adopter file

* add gopher slack invite link

* slightly rephrase nightly image section

* add sponsor request for action

* better formatting for contributor wall

* add longer wait time for stale PRs and issues and allow for exemption through bug and high-priority labels

* apply review suggestion

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

---------

Co-authored-by: Joel Speed <Joel.speed@hotmail.co.uk>

docs/docs/welcome.md

@@ -20,4 +20,4 @@ A list of changes can be seen in the [CHANGELOG](https://github.com/oauth2-proxy
 
 ## Architecture
 
-![OAuth2 Proxy Architecture](/img/architecture.svg)
+![OAuth2 Proxy Architecture](/img/simplified-architecture.svg)

docs/versioned_docs/version-7.6.x/welcome.md

@@ -20,4 +20,4 @@ A list of changes can be seen in the [CHANGELOG](https://github.com/oauth2-proxy
 
 ## Architecture
 
-![OAuth2 Proxy Architecture](/img/architecture.svg)
+![OAuth2 Proxy Architecture](/img/simplified-architecture.svg)

docs/versioned_docs/version-7.7.x/welcome.md

@@ -20,4 +20,4 @@ A list of changes can be seen in the [CHANGELOG](https://github.com/oauth2-proxy
 
 ## Architecture
 
-![OAuth2 Proxy Architecture](/img/architecture.svg)
+![OAuth2 Proxy Architecture](/img/simplified-architecture.svg)