Better documentation for typedthreads module (#23483)

Added a second example inside the `typedthreads` file.

Also, add a more detailed introduction. When Nim is one's first
programming language, a short explanation of what a thread is might not
hurt.

For reference, the thread documentation of other languages looks like
this:
- https://en.cppreference.com/w/cpp/thread/thread
- https://doc.rust-lang.org/std/thread/

The documentation of a module is the first place one will look when
using a standard library feature, so I think it is important to have a
few usable examples for the main modules.

This is the example added
```nim
import locks

var l: Lock

proc threadFunc(obj: ptr seq[int]) {.thread.} =
    withLock l:
        for i in 0..<100:
            obj[].add(obj[].len * obj[].len)

proc threadHandler() =
    var thr: array[0..4, Thread[ptr seq[int]]]
    var s = newSeq[int]()
    
    for i in 0..high(thr):
        createThread(thr[i], threadFunc, s.addr)
    joinThreads(thr)
    echo s

initLock(l)
threadHandler()
deinitLock(l)
```

Sharing memory between threads is very very common, so I think having an
example showcasing this is crucial.

---------

Co-authored-by: Andreas Rumpf <rumpf_a@web.de>

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