doc: Update OpenBSD build docs, added resources for pkg_add and doas

[jessebarton]

Updated the preparation section to mirror the freebsd build doc.
Gave resources for pkg_add and information on using doas the built in tool on OpenBSD for running commands as other users.

[shaavan]

ACK cc7e5e1

The updated documentation helps the reader clearly understand the purpose of using the pkg_add command. The updated documentation gives the user ample resources to know how to install software on OpenBSD in general and how to use this software as a user other than root.

I have checked that the added links point to the correct webpage.

[fanquake]

~0

Our build documentation really isn't somewhere we should be documenting how to use sudo, doas, or the basics of how to install a package on whichever operating system the user has installed. I would almost prefer we go the other way, and remove the "how to use sudo" and "how to use your package manager" info from the FreeBSD docs.

[jessebarton]

~0

Our build documentation really isn't somewhere we should be documenting how to use sudo, doas, or the basics of how to install a package on whichever operating system the user has installed. I would almost prefer we go the other way, and remove the "how to use sudo" and "how to use your package manager" info from the FreeBSD docs.

That's a fair point. I suppose there is assumed knowledge of the operating system the user has installed and no need for extra information added to the build docs that's not necessary for someone that already understands installing packages on that particular OS.

I can strip down the FreeBSD docs to match the current OpenBSD docs so as to not provide extra info on basic OS administrative steps.