Create unified Bitcoin design principles page

[danielnordh]

Adds new page for unified principles following issue #116.

Todo:

• Get feedback and reviews of page content
• Make sure nav order of all (other) pages are correct
• Remove Principles from Private Key Management chapter
• Check links to/from other pages in the guide

[pavlenex]

Solid list and principles. Some early light feedback.

[pavlenex]

I did a final look at the content in-depth. Structure-wise, great work, and I think you included the most important principles and emphasized their importance.

I've suggested a few minor typos and grammar-related fixes I've noticed, as well as 2-3 attempts to simplify the structure of the sentence, which is up to you to accept or ignore, it's just a personal preference.

[pavlenex]

tACK.

This one is good to get merged IMO 🚀

[GBKS]

tACK.

Two small follow-up tasks I can think of:

• Add more visuals to represent the different principles (sounds like a fun and challenging exercise)
• Cross-link to principles from various sub-pages

[pavlenex]

@danielnordh @GBKS For the follow-up tasks, if you'd like we can open a separate issue to keep a track of it.

🚀 🚀