Converted main site pages to Markdown format.

src/site/apt/building.apt

src/site/apt/configuration.apt

src/site/apt/contact.apt

src/site/apt/contributing.apt

src/site/apt/downloads.apt

src/site/apt/guidelines.apt

src/site/apt/sources.apt

src/site/apt/training.apt

src/site/markdown/building.md

src/site/markdown/configuration.md

src/site/markdown/contact.md

src/site/markdown/contributing.md

src/site/markdown/downloads.md

src/site/markdown/guidelines.md

src/site/markdown/index.md

src/site/markdown/sources.md

src/site/markdown/training.md