🧹 [v3 Maintenance]: Update docs to reflect fiber.Ctx struct to interface change

[grivera64]

Maintenance Task Description

As v3 introduces a new interface for fiber.Ctx in preference over the current struct *fiber.Ctx in v2, the documentation should also reflect this change. Currently, some v3 docs still refer to *fiber.Ctx.

For example:

• /docs/api/ctx.md in lines 20-23
• /docs/api/app.md in line 96

This would involve replacing all uses of the *fiber.Ctx struct to instead use the fiber.Ctx interface in the /docs/ folder.

Impact on the Project

This update will help people in the v2 -> v3 migration process when the full release is ready. This also helps avoid confusion for early testers who are using the docs as a reference point.

There's a potential risk that we could be doing this too early, as there may be more complicated changes besides *fiber.Ctx -> fiber.Ctx in certain v3 components of Fiber. This could cause requiring double the work, when all of the changes can be made at once in a single PR.

Additional Context (optional)

This serves as a smaller issue to split the progress of the following larger issues' goals:

#2712
#2713

Checklist:

• I have confirmed that this maintenance task is currently not being addressed.
• I understand that this task will be evaluated by the maintainers and prioritized accordingly.
• I am available to provide further information if needed.

[gaby]

Fxied via #2880