Bot should not @ mention/tag usernames

[JoshuaKGoldberg]

Describe the bug

Adding someone to an AllContributors table involves quite a few @ mentions. At least:

1. Asking the allcontributors bot generally involves an @
2. The PR created by the bot includes an @

This creates notifications noise. Some users -IME folks newer to open source- get excited about it and appreciate being called out. But others -IME folks who've done open source for a while- don't appreciate having extra notifications sent their way. Especially if it keeps happening.

Expected behavior

I don't think there needs to be explicit @-based mentions/tags of users in the app bot's flow. Issues/PRs that have allcontributors commands are already visible to those contributors.

Proposal: how about removing usage of @ from docs, and making sure the app works without it?

As in, this change to suggested bot request commands:

- @allcontributors add @example-username for bug
+ @allcontributors add example-username for bug

And this change to the PR text it creates:

- @allcontributors add @example-username for bug
+ @allcontributors add example-username for bug

Additional context

This is something of a sibling issue to #478: which would allow users to opt out of being included altogether.

Example comments where I've been asked to stop spamming a fellow open source maintainer:

• docs: add fisker as a contributor for infra JoshuaKGoldberg/ts-api-utils#591 (comment)
• docs: add fisker as a contributor for bug JoshuaKGoldberg/prettier-plugin-curly#847 (comment)

Edit: we can see from e.g. JoshuaKGoldberg/all-contributors-auto-action#1102 (comment) that the bot can be told to add someone from a comment missing the @. But both the response comment mentioning the PR (JoshuaKGoldberg/all-contributors-auto-action#1102 (comment)), as well as the PR itself (JoshuaKGoldberg/all-contributors-auto-action#1103), include the @. So this is more than just a docs change.