Became a speaker button

[paulacianelli]

As a developer interested in speaking at a ruby.uy meetup,
I want a dedicated page that explains what's expected and lets me reach an organizer privately
on Slack,
so that I can propose a talk without guessing or writing a cold email.

Background:
Today there is no speaker-onboarding path on ruby.uy. Prospective speakers either email
org@ruby.uy blind or have to know an organizer personally. The goal is to lower the barrier for
first-time speakers while giving a clear private channel for those who prefer it. Pure-Jekyll
only — no forms, no backend, uses Liquid + data files + existing vanilla JS patterns.

Scope:

1. New page at /propone-una-charla/ with these sections, in order:
   - Warm intro + explicit "first-time speakers welcome" copy
   - Talk formats (lightning 5 min · corta 15–20 min · completa 30–40 min)
   - Suggested topics (kept short and non-restrictive)
   - "¿Preferís hablar con alguien en privado?" — grid of organizer cards: avatar, name, role,
   "Escribime por Slack →" button linking to that organizer's Slack DM URL
   - "¿No estás en el Slack? Unite acá →" fallback link (reuses existing workspace URL from
   _data/socials.yml)
   - mailto:org@ruby.uy fallback with pre-filled subject and body template
2. CTA "Proponé una charla →" placed in:
   - The hero's "Sé parte_" chips (_includes/default_header.html)
   - The footer (_includes/footer.html)
3. Organizer data model: extend _data/people.yml with organizer: true, role:, slack_url: — or
   introduce a separate _data/organizers.yml (decision deferred to implementation).

Acceptance criteria:

• /propone-una-charla/ renders all sections listed in scope
• Responsive at 425 / 768 / 1300 px breakpoints; matches existing palette (#3967D1 blue,
  #F6EEEC cream, #FFC24D yellow) and typography (Syncopate + DM Sans)
• "Proponé una charla →" chip added to the hero's "Sé parte_" section
• Footer link added
• Organizer cards rendered from a data file, filtered by an organizer flag
• Each card shows avatar (GitHub image), name, role, and "Escribime por Slack →" button linking
  to that organizer's Slack DM URL
• "Unite al Slack →" link present under the organizer grid
• mailto: fallback uses ?subject=Propuesta%20de%20charla&body=
• No personal email addresses exposed anywhere on the page
• All copy in Spanish, matching the tone of the rest of the site