Generate GitHub pages from main branch

[geographika]

This pull request publishes any changes to the main branch to a new gh-pages branch which is then available online at https://mapserver.github.io/MapServer-documentation/

It does not seem possible to publish different branches from the same repository to gh-pages, so a branch is used within MapServer-documentation.

Currently the MapServer documentation workflow is to make pull requests to the "main" branch. This triggers a Sphinx build of the docs HTML output.

This pull request then takes this output and copies it to the gh-pages branch. This is then available at https://mapserver.github.io/MapServer-documentation/ where it can be reviewed. In time it would be nice to set this up on a mapserver.org subdomain.

If the update looks good (and is relevant to the current MapServer release) it can be backported to the "live" branch (currently 7-6), which continues to deploy automatically to https://mapserver.org/

If this pull request is merged then GitHub pages can be activated through Settings > Pages:

For testing in the https://github.com/geographika/MapServer-documentation/tree/gh-pages branch I used HTTPS to connect to GitHub rather than SSH used in travis_deploy_website.sh . I've switched it back for this pull request so hopefully it works as it does in the other script.

# git clone --no-checkout --depth=1 https://geographika:$GITHUB_TOKEN@github.com/geographika/MapServer-documentation $destdir/MapServer-documentation
git clone --no-checkout --depth=1 git@github.com:mapserver/mapserver.github.io.git $destdir/MapServer-documentation

In addition a ribbonis added to the main (development) docs output to make it clear they are not the user docs (as discussed at https://lists.osgeo.org/pipermail/mapserver-dev/2021-September/016618.html).

[geographika]

@jmckenna - thanks for merging. Are you able to access the project "Settings" and activate GitHub pages to point to the gh-pages branch?
The master docs should then appear at https://mapserver.github.io/MapServer-documentation/

[jmckenna]

@geographika thanks yes I understand, but the changes caused a Travis error...so at the moment my answer to your question is no, sorry, I'll get back now to examining the error...

[geographika]

@jmckenna - let me know if I can look at the error. Was this when GitHup Pages was enabled?
The build itself at https://app.travis-ci.com/github/MapServer/MapServer-documentation/builds/238776710 seemed to go fine.