Get a perma-link to the most recent stable SMIRNOFF spec page

[j-wags]

Hmm, the proposed change would hard-code this link to the 0.3.0 toolkit release's SMIRNOFF spec page. This could be a problem because tomorrow we're cutting the 0.4.0 toolkit release, which will make substantial changes to this page (and if we hand-correct that, we'll eventually make more changes requiring an update of this link in the future).

RTD is supposed to have a stable branch which corresponds to the latest tagged release, but that's not building for us. I'll try to figure out what's up with that and get us a perma-link to an up-to-date SMIRNOFF spec page.

I'm going to merge this PR, because an outdated link is marginally better than a completely broken one. But I'm making an Issue to tackle this in the future.

Originally posted by @j-wags in #120 (comment)

[dwhswenson]

Would it be better to have it link to latest (current version of default branch) while waiting to get stable working, instead of pinning to an outdated version?

[jchodera]

Some of those posts specifically refer to specific (now outdated) versions of the spec, so it would make sense to think about how we should link to specific snapshots in future.

[jchodera]

FYI, I've fixed #184 by linking to the final topology branch draft spec in cases that referred to the old draft spec, and the latest spec in cases that discuss the latest SMIRNOFF spec version.

cc: #198

[Yoshanuikabundi]

The stable branch now builds! See the link in the OP, and openff-toolkit issue #339

[j-wags]

Awesome. So the URL would be https://open-forcefield-toolkit.readthedocs.io/en/stable/ or, since it's now the default, https://open-forcefield-toolkit.readthedocs.io/