[docs] Fix decode examples and expected output in fast_tokenizers

[samyuktahegde]

What does this PR do?

While working through the Fast Tokenizers documentation to understand encode()/decode(), I observed a few inconsistencies. This PR updates the examples to match the observed behavior and keeps the documentation consistent across the page.

Documentation Changes

Encode and decode section:

• Replaces tokenizer.decode(outputs["input_ids"]) with tokenizer.decode(input_ids)
  as tokenizer.encode() returns just a list of token IDs rather than an object.
• Updates the expected output of tokenizer.decode() to display that it returns a string (instead of a list).

Special tokens section:

• Applies the same corrections to the repeated decode() example by replacing outputs["input_ids"] with input_ids and updating the expected output to a string.

Fixes # (issue)

Code Agent Policy

The Transformers repo is currently being overwhelmed by a large number of PRs and issue comments written by
code agents. These often are low-quality, or fix extremely minor issues that occur rarely or never in practice.
As a result, we're instituting a rule that first-time contributors should not use code agents to submit PRs or issues.
We'd also ask autonomous "OpenClaw"-like agents not to open any PRs or issues.

Issues/PRs from first-time contributors that violate this rule will probably just be closed without review, and we
might block you, especially if you open more than one or appear to be deliberately ignoring this. We especially do not
want new contributors to jump in on random issues to contribute an agent-written fix. This creates lots of noise
for reviewers and other users and will almost certainly get you blocked.

For more information, please read CONTRIBUTING.md.

• (First-time contributors only): I confirm that this PR description and code is not written by an LLM or code agent

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline and the
  Pull Request checks?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes according to the guidelines?
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[github-actions]

---

CI recap

Dashboard: View test results in Grafana
Latest run: 29501835760
Result: success | Grafana metrics are not available yet.

[Rocketknight1]

Yes, this does look cleaner, and it matches the unbatched decoding logic. Thank you!

[samyuktahegde]

Thank you for the review and approval.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.