add reading config, add and update docs

docs/default-max-threads.md

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+# Default `max-threads`
+
+The default `max-threads` uses a simple algorithm to get how many threads.
+
+```rs
+if total_threads >= 32 {
+    return total_threads - 4;
+} else if total_threads >= 12 {
+    return total_threads - 2;
+} else if total_threads >= 3 {
+    return total_threads - 1;
+} else {
+    return total_threads;
+}
+```
+
+i.e. with `total_threads` as the number of threads the CPU(s) has:
+
+- If the CPU has 32 or more threads, it will use all but 4 threads
+- If the CPU has 12 or more threads, it will use all but 2 threads
+- If the CPU has 3 or more threads, it will use all but 1 thread
+- Otherwise, i.e. if the CPU has 1 to 2 threads, it will use all threads
+
+---
+
+Alternative algorithms I tried:
+
+```rs
+if total_threads >= 32 {
+    return total_threads - total_threads.div_ceil(10);
+} else if total_threads >= 12 {
+    return total_threads - 2;
+} else if total_threads >= 3 {
+    return total_threads - 1;
+} else {
+    return total_threads;
+}
+```