improve content.html build logic

[hamish]

Hi,

OpenJUMP's overview page fails to link from the content.html summary because there is no project_description.html file. Instead it goes to the homepage link from the project_definition.html file.

perhaps the solution is to edit the definition.html file to use doc/project_overview.html

Hamish

[hamish]

OpenJUMP overview and quickstart now seems to be all ok.