Fix rendering issues in architecture docs (#2861)
* Fix rendering issues in architecture docs
ASCII art diagrams had bare code fences that some renderers tried to
parse as diagrams. Tag them as `text`. Also escape :: in a Mermaid
stateDiagram label that was causing parse errors.
* Remove training and COG_WEIGHTS from architecture docs
Training in its current form is being removed. COG_WEIGHTS is also
deprecated. Strip both from the architecture docs rather than adding
deprecation noise -- when custom handlers land, the docs get rewritten.
* Convert ASCII diagrams to Mermaid for consistent rendering
ASCII box-drawing diagrams break when the monospace font has slightly
different glyph widths. Mermaid diagrams render cleanly everywhere --
GitHub, mkdocs, VSCode preview.
* Replace block-beta diagrams with flowcharts
block-beta with custom fill colors breaks on GitHub dark mode --
white text on pink/grey backgrounds. Switch to plain flowcharts
with subgraphs, no custom styles.
