Write a "Conventions" page

[HGSilveri]

Write a "Conventions" page to add under the "Fundamental Concepts" section of the Pulser docs. It should mention:

• [x] The different Hamiltonians and respective bases
• [x] The single-qutrit state and the projections into the single-qubit states in the different bases
• [x] Multi-partite state order
• [x] Measurement convention: what's 0 and what's 1 (depending on basis), order in a measurement sample