chore: update CI base branch from main -> next, document publish process

[hydrosquall]

Motivation

• Presently, every update tagged with release to data-explorer would modify the latest channel on NPM. Having a separate pre-release channel allows bugs / features to be tested by those opting in to an experimental version before rolling out to everyone.
• This PR permits every accepted PR to go out to next tag for ease of testing, but allows maintainers to control the rate of outflow to the main latest tag through manual merges from next -> stable.

Changes

• Updates base branch from main to next
• Updates docs (based on notes written for Vega: chore: set up auto for versioning/release management  vega/vega-tooltip#566 )

Background / Notes

• Similar

Original message from Nteract slack channel:

(Process suggestion) - merging into stable to create a production release

I've been setting up build systems for a few projects lately, and noticed a pattern that we might find useful here (storybookjs/storybook is probably the biggest example)

Rather than having main as the base branch, projects are releasing a dev /next prerelease as the default branch. That branch always releases a new version to the next tag on NPM. To make a stable release that updates the latest tag on NPM, you just merge next into your production branch (I recommend calling it stable ).

I think this would be a good pattern to follow in data-explorer too, but wanted to check with others before making the chance. (It would affect modifying the workflows in the .github folder to check for next instead of main as the primary branch). The effect of this is that instead of remembering to apply tags to decide whether or not a PR should be released to NPM upon merge, you can safely approve everything right into the pre-release branch, and do any necessary fixes before it gets merged into stable for an "official" release.

Let me know if you have any questions or concerns about how this would work (I'll leave this open for a week to give people time to see/think about it before doing anything).

Some additional background on the rationale behind this release process:

vega/ts-json-schema-generator#856 (comment)

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[willingc]

Circling back to open source. Thank you!