Move and update "memory model" docs

[gonzalobg]

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This PR moves the "memory model" related docs to a separate page, so that they can be easily linked from the Programming Guide.

It also updates the memory model docs.

It provides a new guarantee that we did not provide before, system-scope atomics are also atomic if the operation:

it is a load or store that affects a naturally-aligned object of sizes 1, 2, 4, or 8 bytes on mapped memory

The scripts to build the site have stopped working, so this PR also updates those.

The site layout is constrained to a 800px body width, which makes tables render poorly. This PR bumps this to 1000px here.

Syntax highlighting within HTML tables seems to be buggy with Jekyll.

[gonzalobg]

@daniellustig please review.

[wmaxey]

@gonzalobg Is this good to get in?

[gonzalobg]

Waiting for review.

[daniellustig]

Looks good to me, aside from the one minor comment I just posted

[gonzalobg]

@wmaxey @daniellustig i think we can just merge this. Having this would simplify the docs of atomic_ref as well.

[daniellustig]

@wmaxey @daniellustig i think we can just merge this. Having this would simplify the docs of atomic_ref as well.

👍