Fix overlaps in OSR schedule and use markdown for talk descriptions

[cmaumet]

This PR fixes issue identified by @cassgvp on the times on the 22nd June socials and 26th June afternoon break and enables markdown for descriptions.

[cmaumet]

@cassgvp - for each talk the description can be updated as here, i.e.:

• adding a pipe "|" after "description: ",
• removing quotes and using "1." rather than "(1)" for list items
• adding < and > around URLs

Are you happy to take it from here as discussed?