docs/README.md: add dev notes for CDSP documentation site

Add developer notes for the CDSP documentation site so that devs can
build the site locally and contribute changes.

Signed-off-by: Stephen Lawrence <[email protected]>

docs/README.md

@@ -1 +1,97 @@
-This directory contains the project documentation including the sources for the online documentation website.
+This directory contains the project documentation including the sources for the online documentation website.
+
+# Central Data Service Playground (CDSP) documentation site
+
+The [CDSP documentation site](https://covesa.github.io/cdsp/) is realized with GitHub Pages. It is generated from
+the files in the ```/docs-gen``` directory tree of this repository.
+The `assets` folder within that tree contains assets such as architectural diagrams that will be referenced from multiple pages. The `content` folder contains the files such as the markdown files that provide the rest of the content.
+The static webpage is generated automatically after every PR merged to master
+and deployed into a branch called `gh-pages`.
+
+## Known limitations
+
+Links in this folder are typically not possible to follow in Github as they refer to locations in the generated documentation
+rather than the source documentation. That could possibly be solved by [Hugo Render Hooks](https://gohugo.io/templates/render-hooks/)
+but it is not trivial to get a solution that works for all types of internal links.
+
+## Guidelines
+
+Never link directly to a specific page at `github.io` - in the future we might want to keep multiple versions of
+documentation and then links must go to the correct version.
+
+## Verifying links
+
+We can use the OSS package `markdown-link-check` to check markdown files. It can be installed like this
+
+```
+sudo apt install nodejs npm
+sudo npm install -g markdown-link-check
+```
+
+What to run:
+```
+cd cdsp
+markdown-link-check *.md
+```
+
+For generated files (Hugo) links can be checked by running the cloud service [W3C Link Checker](https://validator.w3.org/checklink), against the location of the online documentation at `https://covesa.github.io/cdsp/`. It checks against latest master.
+Run the check with the option `Check linked documents recursively` enabled.
+
+
+## Dependencies
+
+The static page is generated with:
+
+- [HUGO](https://gohugo.io/)
+- [Learn Theme](https://github.com/matcornic/hugo-theme-learn)
+
+Please follow the [documentation](https://gohugo.io/documentation/) for installation and further questions around the framework.
+Currently, the HUGO version used for generating VSS documentation is `0.124.0`,
+as controlled by the [buildcheck.yml](https://github.com/COVESA/cdsp/blob/main/.github/workflows/buildcheck.yaml)
+
+
+## Run the documentation server locally
+
+Once hugo is installed please follow the following steps:
+
+### Check that HUGO is working:
+```
+hugo version
+```
+The following outcome is expected:
+
+```
+hugo v0.xx.xx ...
+```
+
+### Clone the submodule containing the theme
+
+Run the following git commands to init and fetch the submodules:
+
+```
+git submodule init
+git submodule update
+```
+
+Reference: [Git Documentation](https://git-scm.com/book/en/v2/Git-Tools-Submodules).
+
+### Test locally with your Hugo server:
+
+Within the repository
+
+```
+hugo server --disableFastRender -D -s ./docs-gen/
+```
+
+Optional ```-D``` include draft pages as well. Afterwards, you can access the
+page under http://localhost:1313/cdsp/.
+
+## Contribute
+
+If you want to contribute, do the following:
+
+1. Change documentation in ```/docs-gen```
+
+1. Test your changes locally, as described above
+
+1. Create Github Pull Request for review