add calendar repositories

[davidtwco]

The compiler team currently use Google Calendar to manage our team calendar, and that's a bit annoying because we can't manage the access controls to that in the same way we do everything else - it requires manually adding team member's Google account (assuming they have one) to the calendar.

As an alternative to this, I've implemented a small utility for generating calendar files from TOML and hosting them in GitHub Pages. This makes it easy to get a link that can be subscribed to in a user's calendar application of choice, and it can be managed using the same workflows and tooling we have for everything else. I've written this in such a way that other teams could easily add their own calendars, so that this becomes a standard mechanism within the project, and that the general calendar generation can be re-used easily by other projects too.

• calendar-generation contains a utility that will generate an iCalendar document from TOML files containing the events. This repository already exists at davidtwco/toml-to-ical and if we are to use it, it should be moved so that I can continue to maintain it as the project uses it, and can be maintained by the rest of the project if necessary.
• calendar contains TOML files and CI configuration to use calendar-generation to generate and then host the calendar documents on GitHub Pages. A draft of this repository exists at davidtwco/rust-calendars-draft, replicating the current events in the compiler team's calendar.

[Mark-Simulacrum]

Not sure if a single calendar repo vs. per team makes sense, but I think we can start here.

[davidtwco]

Links to created repositories for those following along at home:

• https://github.com/rust-lang/calendar-generation
• https://github.com/rust-lang/calendar