- fix link to expressions syntax (upgrade v1912)
- add mirror information
- repos jump directly to /Development
- additional quick links to repo and issues

building.md

@@ -3,6 +3,7 @@
 [![home](/icons/home.svg "wiki home")](/home)
 [![modules](/icons/layers.svg "modules")](/modules)
 [![packages](/icons/package.svg "packages")](/precompiled)
+[![repos](/icons/gitlab.svg "repos")][all-repos]
 
 [[_TOC_]]
 
@@ -154,6 +155,8 @@ example:
 
 Copyright (C) 2020 OpenCFD Ltd.
 
+[all-repos]:    https://develop.openfoam.com/Development/
+
 [latest-build]: https://develop.openfoam.com/Development/openfoam/blob/develop/doc/Build.md
 
 [v2006-build]: https://develop.openfoam.com/Development/openfoam/blob/maintenance-v2006/doc/Build.md