Create sessions overview page

[Kingdutch]

Create a page that contains information about a session. Barcelona example: https://barcelona.rustfest.eu/talks/

The page can be made by creating a sessions component in the src/pages directory.

Contains the speaker component from #6
Links to to the detail pages from #7

Session data can be added by copying the content/sessions/example.md file.

All sessions can be selected with the following query. Speakers are automatically linked.

allSessions: allMarkdownRemark(
    filter: { fields: { collection: { eq: "sessions" } } }
  ) {
    nodes {
      fields {
        speakers {
          frontmatter {
            name
          }
        }
      }
      frontmatter {
        title
        date
        speakers
      }
    }
  }