An unclosed `<docs-step>` or `<docs-card>` produces no error, just wrong
output, so the two cases fixed in the previous commit went unnoticed for
years.
Validate that the paired custom elements balance before parsing. A
mismatch now names the file and the counts instead of silently
swallowing a section.
Move the header id tracking into the renderer context rather than a global state. Since the RendererContext is
initialized for each execution of the marked parser, we can store the context of which header ids have been seen within
a single page there which allows us to run these parse interactions in parallel.
Kam