[Docs] Add installation notice for strict mode

Nobody has claimed this yet.

**Describe the issue**
The installation guide should add a mention that using `strict` type-checking requires making class properties with a `!` or disabling strict checking for property initialization in classes. Elsewise users will face the following error:

> Property 'xy' has no initializer and is not definitely assigned in the constructor

My suggestion (to be added at the end of https://typegraphql.com/docs/installation.html):

> In case of using strict type-checking in `tsconfig.json`, all non-optional class properties are required to use the non-null assertion operator `!`.
>
> As an alternative solution strict checking of property initialization in classes can be disabled:
>
> ```json
> "strictPropertyInitialization": false
> ```

**Are you able to make a PR that fix this?**
I can do if my suggestion is ok.

Open the installation guide at https://typegraphql.com/docs/installation.html and review its ending. Add a notice explaining the strict type-checking requirement for non-optional class properties and the alternative strictPropertyInitialization setting shown in the issue. Done means the guide clearly presents both options and the reported error is addressed.

Written by the indexing model from the issue text.