Remove manual instructions, update postfix instructions

We should fix a few items on the /installation page:

• Now that we have a page documenting the manual install process, we should direct there instead of having a code block specifically for it.
• We should then split up the installation code block with two lines into two blocks of one line each.
• We should then also note that again that installation of postfix may require a few pressed of enter, after the code block.