feat(get_document_id): add new rust exercise

subjects/get_document_id/README.md

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+## get_document_id
+
+### Instructions
+
+Create the following structures which will help you get the `document_id` of the document you need.
+
+- `OfficeOne`: `next_office` as type `Result<OfficeTwo, ErrorOffice>`.
+- `OfficeTwo`: `next_office` as type `Result<OfficeThree, ErrorOffice>`.
+- `OfficeThree`: `next_office` as type `Result<OfficeFour, ErrorOffice>`.
+- `OfficeFour`: `document_id` as type `Result<u32, ErrorOffice>`.
+
+`ErrorOffice` is an `enum` with `OfficeClosed(u32)`, `OfficeNotFound(u32)` or `OfficeFull(u32)`.
+
+The `usize` is the `id` of the office generating the error.
+
+Beside the structures, you must create a **function** named `get_document_id`, and associate it to the `OfficeOne` structure.
+
+This **function** should return the `Result` value in the `OfficeFour` structure or the first `Err` it finds in the chain.
+
+### Expected Function and structures
+
+```rust
+#[derive(Clone, Copy)]
+pub enum ErrorOffice {
+    OfficeClose(u32),
+    OfficeNotFound(u32),
+    OfficeFull(u32),
+}
+
+#[derive(Clone, Copy)]
+pub struct OfficeOne {
+    pub next_office: Result<OfficeTwo, ErrorOffice>,
+}
+
+#[derive(Clone, Copy)]
+pub struct OfficeTwo {
+    pub next_office: Result<OfficeThree, ErrorOffice>,
+}
+
+#[derive(Clone, Copy)]
+pub struct OfficeThree {
+    pub next_office: Result<OfficeFour, ErrorOffice>,
+}
+
+#[derive(Clone, Copy)]
+pub struct OfficeFour {
+    pub document_id: Result<u32, ErrorOffice>,
+}
+
+impl OfficeOne {
+    pub fn get_document_id(&self) -> Result<u32, ErrorOffice> {}
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use get_document_id::*;
+
+fn main() {
+    let office_ok = OfficeOne {
+        next_office: Ok(OfficeTwo {
+            next_office: Ok(OfficeThree {
+                next_office: Ok(OfficeFour {
+                    document_id: Ok(13),
+                }),
+            }),
+        }),
+    };
+    let office_closed = {
+        OfficeOne {
+            next_office: Ok(OfficeTwo {
+                next_office: Err(ErrorOffice::OfficeClose(13)),
+            }),
+        }
+    };
+
+    match office_ok.get_document_id() {
+        Ok(id) => println!("Found a document with id {}", id),
+        Err(err) => match err {
+            ErrorOffice::OfficeClose(_) => println!("Error: office closed!"),
+            ErrorOffice::OfficeNotFound(_) => println!("Error: office not found!"),
+            ErrorOffice::OfficeFull(_) => println!("Error: office full!"),
+        },
+    };
+    match office_closed.get_document_id() {
+        Ok(id) => println!("Found a document with id {}", id),
+        Err(err) => match err {
+            ErrorOffice::OfficeClose(_) => println!("Error: office closed!"),
+            ErrorOffice::OfficeNotFound(_) => println!("Error: office not found!"),
+            ErrorOffice::OfficeFull(_) => println!("Error: office full!"),
+        },
+    };
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+Found a document with id 13
+Error: office closed!
+$
+```