core: add md files howto layout

howto.md

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+
+# Backend internal interview (rust) - improved
+
+## How to used
+
+**cargo run --** can be replaced for generated binaries with
+ 
+- target/debug/be-technical-interview-rust (for debug build)
+- target/release/be-technical-interview-rust (for release build)
+
+### For help
+
+```bash
+cargo run -- -h
+```
+
+### For input file **input.txt**
+
+```bash
+cargo run -- -i input.txt
+```
+
+### In quiet mode
+
+```bash
+cargo run -- -i input.txt -q
+```
+
+if **output path** is provided, results will be saved in provided path,<br>
+without **quiet mode** info is printed to terminal
+
+```bash
+cargo run -- -i input.txt -o /tmp/output.txt
+```
+
+### Use config path. **Batch processing in parallel** (one thread for each target) 
+
+```bash
+cargo run -- -c config.toml
+```
+
+### Config file content
+
+```toml
+be_quiet = false
+[[targets]]
+input = "input.txt"
+
+[[targets]]
+input = "input_2.txt"
+```
+
+If **output** path is provided, **out_overwrite** can be used (true or false) to append conten or rewrite.
+
+By default:
+
+- **out_overwrite** is **true**
+- *reg_exp** is set as
+
+```rust
+pub const DEFAULT_REG_EXP: &str = r"(\d+) (\w+) (\d+)";
+```
+
+**reg_exp** can be provided but as a **regex** expresion from **string**, if it can not be converted parser exit.
+
+> [!CAUTION]
+> Command line options have precedence over **config path** settings. <br>
+> Be careful with the combinations

layout.md

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+
+# Backend internal interview (rust) - improved  
+
+Files layout 
+
+```markdown
+.
+├── ABOUT.md           [Verification](ABOUT.md)
+├── Cargo.lock
+├── Cargo.toml
+├── CHANGES.md         [Changes](CHANGES.md)
+├── Home-Task_exercise_interview_Rust.pdf
+├── LICENSE
+├── README.md 
+├── assets             [Assets files for rustdoc](assets)
+│   ├── doc.css
+│   ├── header.html
+│   └── howto.md
+├── howto.md           [How to use](howto.md)
+├── input.txt
+├── input_2.txt        [Second input for parallel](input2.txt)
+├── layout.md          [Files layout](layout.md)
+├── output_expected.txt
+└── src                [source code](src)
+   ├── defs     
+   │   ├── cli.rs
+   │   ├── config.rs
+   │   ├── metric_data.rs
+   │   └── metrics.rs
+   ├── defs.rs
+   ├── main.rs
+   ├── metrics_consumer.rs
+   └── tests.rs
+```
+
+**.cargo** for **rustdoc** documentation build.
+**.gitignore** to exclude paths like **target**
+