Streamline counter, wCCD and voting smart contract tutorials

Purpose

Streamline and update the tutorials to make them simpler and easier to follow.

Rewrote counter tutorial to improve the wording and simplify the code.
- Many subpar VSCode screenshots that felt unnecessary were removed.
- The code was simplified to use a single update function rather than separate increment and decrement functions. A single error variant OverflowError is used instead of separate errors for the two previous functions.
- Added instructions for how to use Concordium's testnet node to go through the tutorial, which makes it much easier to follow.
Slightly edited the wCCD tutorial, including merging the last part with the previous part since it is so short.
Some cleaning up for the voting tutorial, including making it adhere to the new simplified version, see https://github.com/Concordium/concordium-rust-smart-contracts/pull/440.
Removed some redundant/repeated sections in the piggybank tutorial and elsewhere.

[x] My code follows the style of this project.
[x] The code compiles without warnings.
[x] I have performed a self-review of the changes.
[x] I have documented my code, in particular the intent of the hard-to-understand areas.
[x] (If necessary) I have updated the CHANGELOG.