feat: align discovery with mppx

[stevencartavia]

Simplified to_llms_txt_with() to match mppx toLlmsTxt(): title heading, description blockquote, service list, OpenAPI discovery link.

Part 2 of 3:

• Step 1: add service metadata fields 164
• **Step 2: align discovery with mppx (this PR)
• Step 3: add generate_openapi() + wire GET /openapi.json route

[brendanjryan]

@stevencartavia -- what is the motivation for this change? I would prefer we match the equivalent shape and functionality mppx if possible instead of adding features only to the rust SDK unless explicitly needed

[stevencartavia]

I've removed the features that were deleted in (wevm/mppx@b4e1a3d) (wants_markdown, markdown renderers, DiscoveryResponse::Markdown) and simplifies to_llms_txt_with() to match mppx's toLlmsTxt(). Follow-up PR will add generate_openapi() matching mppx's generateProxy().

[brendanjryan]

lgtm!