Developer guide needs adjustments

to account for

• Conda recipes
• Don't mention conda-forge
• How to test packages locally, including conda-build or buildout-based setups
• Needs fixing: https://www.idiap.ch/software/bob/docs/bob/bob.extension/master/additional.html#continuous-integration-ci
• New bob-devel package which does not install anything by default
• Merge the "developing existing packages" guide into bob.buildout bob.buildout!29 (comment 26098)