Add layout for slides

[peterdesmet]

See https://echinopscis.github.io/blog/2022/tdwg-conference-lightning-talk/ by @nickynicolson in which slide information is added to front matter:

slides:
  - img: /assets/images/tdwg2022-ltd14/Slide1.PNG
    img_alt: "Open science tools: Supporting hands-on creation of the “digital extended specimen”"
    notes: |
      Work is a collaboration between [Nicky Nicolson](https://www.kew.org/science/our-science/people/nicky-nicolson) (NN) and [Eve Lucas](https://www.kew.org/science/our-science/people/eve-lucas) (EL) both from [Kew Science](https://www.kew.org/science)
  - img: /assets/images/tdwg2022-ltd14/Slide2.PNG
    img_alt: "Explain personal and institutional contexts"
    notes: |
      Personal context: **NN** transitioned from software development into research, and is interested in how software development practices can be used in research. Institutionally, we have a commitment to accelerate taxonomy using digital practices, and we started a collaborative project to explore this, using institutional knowledge from the e-taxonomic approaches, but exploring what technical advances could be used in e-taxonomy today. 
    video_offset: 0m31s

The slides layout then renders on the page as (including option to jump to video)

Observations:

• Is it better to have this info in front matter or as a _data/file.yml ? Since it is for a single page, it might be good to keep it together, but doesn't follow conventions.
• Would change layout so that text appears under (rather than next to slide) on small screens.