Errata pages from GitHub

[plehegar]

The idea here is, based on GitHub labels from issues/pull requests, we could show the list of errata of a document.

[plehegar]

We need:

• documentation for errata management: Create errata.md Guide#15
• a tool that is capable of displaying the list of errata given a static REC
• monitoring and reporting system(s) to oversee our errata

[swickr]

@iherman created some tooling for WGs for which he had been Team Contact: see
Using Github issues to handle errata in Recommendations

[plehegar]

I raised a bunch of issues for @iherman to look at.

[plehegar]

this is still ongoing but we're (slowly) deploying https://github.com/w3c/display_errata .