[ci] Docs comment bot should update comments in-place

[driazati]

Posting a new comment generates a notification which I don't think we want beyond the first one from github-actions since it doesn't carry much signal. The docs-bot workflow should search the PR for existing comments from the github-actions bot (possibly marked with a markdown comment or something like <!--- docs-bot-comment -->) and update that comment instead of posting a new one.

For example, on #11666, the bot would update comment 1 instead of posting comment 2

cc @Mousius @areusch @gigiblender

[gigiblender]

@driazati I agree with you. I can take a look at this issue.

[gigiblender]

#11909