Add stable docs badge (#105)

* add badge for stable docs to README and docs

* add documentation section with links to README

* Update README.md

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

* Update README.md

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

---------

Co-authored-by: Michael Schlottke-Lakemper <michael@sloede.com>

Author: benegee

README.md

@@ -1,5 +1,6 @@
 # libtrixi
 
+[![Docs-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://trixi-framework.github.io/libtrixi/stable)
 [![Docs-dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://trixi-framework.github.io/libtrixi/dev)
 [![Build Status](https://github.com/trixi-framework/libtrixi/workflows/CI/badge.svg)](https://github.com/trixi-framework/libtrixi/actions?query=workflow%3ACI)
 [![Coveralls](https://coveralls.io/repos/github/trixi-framework/libtrixi/badge.svg)](https://coveralls.io/github/trixi-framework/libtrixi)
@@ -251,6 +252,11 @@ this. If you skip this step, everything will work as usual, but some things migh
 slightly slower.
 
 
+## Documentation
+Documentation for the current release can be found at https://trixi-framework.github.io/libtrixi,
+and for the current development version at https://trixi-framework.github.io/libtrixi/dev.
+
+
 ## Authors
 Libtrixi was initiated by
 [Benedict Geihe](https://www.mi.uni-koeln.de/NumSim/)

docs/src/index.md

@@ -1,5 +1,6 @@
 # libtrixi
 
+[![Docs-stable](https://img.shields.io/badge/docs-stable-blue.svg)](https://trixi-framework.github.io/libtrixi/stable)
 [![Docs-dev](https://img.shields.io/badge/docs-dev-blue.svg)](https://trixi-framework.github.io/libtrixi/dev)
 [![Build Status](https://github.com/trixi-framework/libtrixi/workflows/CI/badge.svg)](https://github.com/trixi-framework/libtrixi/actions?query=workflow%3ACI)
 [![Coveralls](https://coveralls.io/repos/github/trixi-framework/libtrixi/badge.svg)](https://coveralls.io/github/trixi-framework/libtrixi)