CSSTUDIO-2487 Add documentation when building the Phoebus package in 'phoebus-product' using maven

[abrahamwolk]

This pull request adds the Phoebus documentation (if it has been built, by for example running mvn -Dsphinx -Nverify in the phoebus source directory) to the Phoebus package that is created when building phoebus-product using maven.

The intention is to facilitate the inclusion of the documentation when deploying the built Phoebus package.

If the documentation has not been built, then it is not included.

[georgweiss]

While this seems to do the right thing - i.e. to include out put of docs/build/html - I'm not sure if a mvn clean should delete the contents of docs/build.

@shroffk, can you share your input? This PR is about being able to include the output of docs build into the tar ball in phoebus-product/target.

[abrahamwolk]

I'm not sure if a mvn clean should delete the contents of docs/build.

Do you mean that with this pull request, running mvn clean in phoebus-product deletes the contents of ../docs/build/html? If so, I don't think it does: running mvn clean in phoebus-product removes only the directory target, as far as I can see.