[w3c/uievents] Setup spec-prod CI (#297)

This requires adding `main` branch as the default one instead of the current `gh-pages`. Demo: https://github.com/saschanaz/uievents/tree/gh-pages



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### :boom: Error: 500 Internal Server Error :boom: ###

[PR Preview](https://github.com/tobie/pr-preview#pr-preview) failed to build. _(Last tried on Mar 20, 2021, 10:04 PM UTC)_.

<details>
<summary>More</summary>


PR Preview relies on a number of web services to run. There seems to be an issue with the following one:

:rotating_light: [Spec Generator](https://www.w3.org/2015/labs/) - Spec Generator is the web service used to build specs that rely on ReSpec.

:link: [Related URL](https://labs.w3.org/spec-generator/?type=respec&url=https%3A%2F%2Frawcdn.githack.com%2Fw3c%2Fuievents%2F9a5541a03ddbd16f86d3f5ba8e5545c4d68eea13%2Findex.html%3FisPreview%3Dtrue)

```
📡 HTTP Error 404: https://rawcdn.githack.com/w3c/uievents/9a5541a03ddbd16f86d3f5ba8e5545c4d68eea13/index.html

```

_If you don't have enough information above to solve the error by yourself (or to understand to which web service the error is related to, if any), please [file an issue](https://github.com/tobie/pr-preview/issues/new?title=Error%20not%20surfaced%20properly&body=See%20w3c/uievents%23297.)._
</details>

You can view, comment on, or merge this pull request online at:

  https://github.com/w3c/uievents/pull/297


-- Commit Summary --

  * Add CI
  * Fix validation errors from spec-prod

-- File Changes --

    A .github/workflows/ci.yml (20)
    M build.py (15)
    D index.html (12539)
    M sections/event-types.txt (4)
    M sections/legacy-event-types.txt (10)
    M sections/legacy-key-attributes.txt (6)

-- Patch Links --

https://github.com/w3c/uievents/pull/297.patch

https://github.com/w3c/uievents/pull/297.diff


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