DOC: regenerate examples and small fixes

[bocklund]

Examples could be regenerated and some low hanging fruit issues could be tackled (few lines of changes maximum for each item):

• Add v.N: 1 as a condition for all equilibrium calls and N=1 for all calculate calls.
• Make sure xarray output is using the newer rich HTML output, which is what users will see
• Binary phase diagram examples: when creating a figure in the notebook, use a higher DPI (maybe 150 or 200?) instead of increasing the figsize
• "Calculating Energy Surfaces of Binary Systems" - do the calculate step for all phases in one call and use xarray calc_res.where(calc_res.Phase == phase_name) syntax for masking in the loop
• "NP vs. T" - use the broadcast_like functionality of xarray, similar to the cookbook example for "NP vs. T" [here]
• Prefer xarray indexing and broadcasting to numpy indexing wherever possible (see, for example the "NP vs. T" cookbook example)
• Change the reference state of the convex hull enthalpy of formation example to be at 298.15 K.
• Activity example: Use v.R instead of hardcoded 8.315
• Activity example: change the title of the plot to "1023 K" to match the data

Related: it would be good to be running the docs notebooks as part of our CI to catch any docs regressions. The website build could regenerate the docs on-the-fly in each build, but it probably isn't necessary.