docs(*): update CONTRIBUTING.md and create DEVELOPERS.md

CONTRIBUTING.md
- focus on basic info about issues and pull requests for new contributors
- move development heavy info to DEVELOPERS.md + add links
- remove outdated info

DEVELOPERS.md
- contains info about project setup, coding rules, and commit message guidelines from CONTRIBUTING.md
- add and update info about writing docs from Wiki
- add info about development setup from docs contribute.md
- add info about running tests on Saucelabs / Browserstack

Closes #7303
Closes #9444 
Closes #16297

Author: Narretz

.github/ISSUE_TEMPLATE.md

@@ -11,7 +11,7 @@ IF YOU DON'T FILL OUT THE FOLLOWING INFORMATION WE MIGHT CLOSE YOUR ISSUE WITHOU
 <!-- (check one with "x") -->
 - [ ] bug report
 - [ ] feature request
-- [ ] other (Please do not submit support requests here (see above))
+- [ ] other <!--(Please do not submit support requests here - see above)-->
 
 **Current behavior:**
 <!-- Describe how the bug manifests / how the current features are insufficient. -->
@@ -27,7 +27,8 @@ https://plnkr.co or similar (you can use this template as a starting point: http
 -->
 
 **AngularJS version:** 1.x.y
-<!-- Check whether this is still an issue in the most recent stable or in the snapshot AngularJS version (https://code.angularjs.org/snapshot/) -->
+<!-- Check whether this is still an issue in the most recent stable or in the snapshot AngularJS
+version (https://code.angularjs.org/snapshot/) -->
 
 **Browser:** [all | Chrome XX | Firefox XX | Edge XX | IE XX | Safari XX | Mobile Chrome XX | Android X.X Web Browser | iOS XX Safari | iOS XX UIWebView | iOS XX WKWebView ]
 <!-- All browsers where this could be reproduced (and Operating System if relevant) -->

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,4 @@
+<!-- General PR submission guidelines https://github.com/angular/angular.js/CONTRIBUTING.md#submit-pr -->
 **What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)**
 
 
@@ -15,9 +16,9 @@
 
 
 **Please check if the PR fulfills these requirements**
-- [ ] The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
-- [ ] Tests for the changes have been added (for bug fixes / features)
-- [ ] Docs have been added / updated (for bug fixes / features)
+- [ ] The commit message follows our [guidelines](../DEVELOPERS.md#commits)
+- [ ] Fix/Feature: [Docs](../DEVELOPERS.md#documentation) have been added/updated
+- [ ] Fix/Feature: Tests have been added; existing tests pass
 
 **Other information**: