added index.html to .gitignore

[himorin]

to prevent events similar to #1184 (review)

(do we need to keep node/npm related lines used(??) for TravisCI?)

[toji]

We may need to revert this. The reason why this wasn't part of the ignored files previously is because our hook to publish the rendered spec to the gh_pages branch was failing to commit the index.html if that was there. We should verify whether or not that's still happening, and if so either revert this change or update our Travis scripts to remove it prior to pushing the published changes.

[Manishearth]

ah oops

The publish step should perhaps force add it

[himorin]

We might need smarter one, but current deploy script explicitly excludes gitignore, so I believe we will not have that again.

https://github.com/immersive-web/webxr/blob/main/.github/workflows/deploy.yml#L18

   run: mkdir deploy && rsync -av --exclude=.git --exclude=.gitignore --exclude=deploy . deploy/

[toji]

Ah, good to know! Thank you!