capstan: Embed the link to Capstanfile documentation in appropriate section
capstan: Remove links to the mailing list and twitter

These are not capstan-specific
capstan: Rename "compatibility" section into "installation"
Make second level headers closer to the content below than to the content above
Add margin to <pre>
run-locally: do not show the whole README link
run-locally: describe how to use capstan to run OSv

For a first-time user using Capstan will be much easier than building
from sources. I think we should promote that approach. If someone
want's to run form sources, there's OSv README file.