[Readme] Add hint on building docs (#7389)

* add hint on building docs

* Update README.md

* Update README.md

* Expand doxygen README

README.md

@@ -160,6 +160,12 @@ The following flags (with their non-default values) can be passed to `cmake`:
 | `-DUPDATE_TRANSLATIONS=1`  |  Configure `make` to update the translation .ts files for new strings in the source code<br> **Note:** `make clean` will remove the .ts files |
 | `-DTEST=1`                 | Enable regression tests<br> **Note:** `make test` to run tests, *googletest* will be downloaded if not available                              |
 
+> [!TIP]
+> Build code documentation ([Doxygen](https://www.doxygen.nl)) according to our [configuration](https://github.com/Cockatrice/Cockatrice/blob/master/Doxyfile) into `/docs`:
+> ```bash
+> doxygen Doxyfile
+> ```
+
 
 # Run
 

doc/doxygen/README.md

@@ -1,3 +1,14 @@
+## Building Doxygen Documentation
+
+We use [Doxygen](https://www.doxygen.nl) for our code documentation.  
+After installing the tool, you can build a HTML webpage into `/docs` folder according to our [configuration](https://github.com/Cockatrice/Cockatrice/blob/master/Doxyfile):
+```bash
+doxygen Doxyfile
+```
+Open the `index.html` file in the `/docs/html/` folder to view and use it.  
+There is also an online version we host at the Cockatrice webpage: [cockatrice.github.io/docs](https://cockatrice.github.io/docs)
+
+
 ## Doxygen Documentation Theme
 
 Required changes to the `Doxyfile` config from the [theme docs](https://jothepro.github.io/doxygen-awesome-css/index.html#autotoc_md16) to make [doxygen-awesome-css](https://github.com/jothepro/doxygen-awesome-css) work: