docs(install/3ds): Improve preamble section

[angeloanan]

This PR aims to clean up the preamble section of the 3DS guide.

Specifically, it inlines the multiple cautions and tips by turning it into a dedicated section, as well as extending the guide's introductory statement by mentioning what the guide will do.

Detailed changes

• Inlined multiple tips and caution to be a dedication section "Things to know before you start" section
• Introduced an opening statement on how to connect to pretendo and what the guide will achieve
• Extended the guide steps to add the "Signing into your PNID" section

[angeloanan]

... and could actually be worse for the experience as it makes it more likely that readers would skim past important information

I would argue that the reverse thing would be true. It is not intuitive to have an all-caps back-to-back callout on the very top of the screen, especially when the same information could just be conveyed as a normal section of a guide that the user firstly read.

[jonbarrow]

The information can be written as plain text, but again as stated the issue is that that runs a greater risk of being missed by users. Having brightly colored, eye catching, callout blocks has a far greater chance of being seen by users. This is also the standard practice for wikis/guides, which I left examples of in the review comment

It's less about where the information is at and more about the removal of the callouts. It doesn't HAVE to be at the top of the page, the callouts just get placed where they make sense (which in this case is at the top of the page)