Introduce translation guidelines

[suryabdev]

An old PR #794 removed the translation guide section because it pointed to an invalid link.
It may have be a leftover section from when the smolagents code was in the transformers repo.

The transformers repo includes a TRANSLATING.md file that outlines a nice process.

TLDR ☝️

1. An issue (if it does not exist) is created listing the files that should be translated for each target language.
2. Once a file is translated, it is checked off the list.
3. Another native speaker approves translation PRs after proofreading (Example). Maintainers then merge the change.
4. The issue acts as tracker.

So far, smolagents has Chinese, Korean and Hindi translations (Spanish soon #1806!). Maybe we could follow the same process as transformers.

I created a draft PR #1811 to copy over the TRANSLATING.md file and added a GitHub issue template, as done in transformers.

[suryabdev]

cc: @albertvillanova for your thoughts