Add speaker portal/guidelines page

[rebkwok]

Transferring from 2024 repo
PyconUK/2024.pyconuk.org#4

Instructions for speakers and accessibility guideline, to include things like:

• slide preparation
  • font size
  • colours (contrast, colour blindness etc)
  • providing a link to slides as the first slide, so audience can follow along (useful for slides with detail, and for people who can't see the slides as well as others)
• providing draft slides in advance (and why we need them)
• on-site prep
  • checking in with session chairs
  • telling welcome desk you're here
  • checking presenting devices work