feat(standards): add Events Format NEP

[telezhnaya]

Closes #297

In the separate branches, I will rewrite NFT NEP-171 a bit and add events section to FT NEP-141 standard.

[render]

Your Render PR Server URL is https://nomicon-pr-298.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-c6t0tgnh8vl74idv7khg.

[zcstarr]

I think this looks great, I'll pull this in to the events within the mt as well. I'm excited to just reference this!

[frol]

How about we have a section with a simple log! usage, where we basically just show that you can emit an event even with a plain string?

Just an illustration of what I mean:

use near_sdk::log;

// ...
log!(
    r#"EVENT_JSON:{"standard": "nep171", "version": "1.0.0", "event": "nft_mint", "data": [{"owner_id": "{}", "token_ids": ["{}"]}]}"#,
    owner_id,
    token_id
);

The drawback here is that this might lead to the code that is prone to "injection"

[telezhnaya]

I feel we also need to expand our examples. Tried to do that by myself, gave up, created the issue 😅
near/near-sdk-rs#685
If we do that in one day, I surely want to include the link to this example here