add "talks" section

[boegel]

We should try and highlight presentations related to EESSI in our docs, ideally with the recording of the talk embedded on the page.

We could aim to list all talks from 2025 onwards, and also list a couple of "key" talks from the past (like the CVMFS tutotial, EESSI EPICURE tutorial, etc.)

I envision this as a talks.md landing page which just lists the talks in reverse order (most recent on top), and then a dedicated page for each talk, like:

### 2025

* [example talk (6 March 2025)](talks/2025/03/example.md)