Replace BIP 77 mermaid diagrams with ascii diagrams

[DanGould]

Updated sequence diagrams to use text format instead of mermaid syntax responding to @achow101's comment

I cargo cult'd the RFC Rules:

“How are images handled for the plain text version of an RFC?”
The RFC Editor will accept both ASCII art and SVG. If only ASCII art is provided, it will be included in all publication formats. If ASCII art and SVG are both provided, ASCII art will be included in the plain text, and SVG in all other outputs. A note indicating alternative artwork is available is strongly advised. If only SVG is provided, a URI will be included in the plain-text publication format pointing to the HTML version. All artwork and figures should have a complete written description to support assisted reader technology.

see: https://www.rfc-editor.org/rse/format-faq/

Since BIPs don't publish html/pdf renders, ASCII art seems like the right choice to render everywhere. Since normative prose is already provided, I chose not to include a written description of the diagrams to support assisted reader tech.

cc @nothingmuch

[ajtowns]

Rather than downgrading to ascii art, maybe include the mermaid source in a bip-0077 directory and generate an svg from it (either using mermaid-cli or copy/pasting into the live editor and exporting?) At the very least, mermaid source is easier to edit than ascii art.

[DanGould]

This was something we bikeshedded even in the original PR and is not the most consequential. I am revisiting only because of a request. ascii art is about as simple as possible, and these diagrams are ancient at this point and really don't need to change. LLMs will produce ascii art from mermaid source, which is now in the commit history. I'd like to go ahead with this as-is for simplicity unless there's a really really good reason to change again.