Translating contributor onboarding

### Describe the problem

New users of Weblate are often overwhelmed by the UI and don't understand all the features. It's not realistic to expect reading the documentation, so we might need some in-application guidance to help users understand the UI and features.

### Solution brainstorm

* Giving a quick walkthrough to the users when they first land on the translate page.
* Explaining searching when using it for the first time.
* There might be more areas that are not obvious to use for users hitting Weblate for the first time.
* There is even more to tell to people using Weblate for a long time but not being aware of some features.
* On the other side, we would rather not make this annoying.

### Describe alternatives you have considered

_No response_

### Screenshots

_No response_

### Additional context

_No response_

Start by reviewing the translate page and the first-use searching flow described in the issue. Clarify which onboarding guidance is in scope, how it should avoid becoming annoying, and define the user-facing behaviors and tests that would show the guidance works.

Written by the indexing model from the issue text.