feat(rust/signed-doc): RefRule multiple boolean flag

[Mr-Leshiy]

Description

• Added multiple: bool to the RefRule structure definition.
• Updated doc_refs_check validation function, with the corresponding check.
• Updated ContetnRule::Templated, ReplyRule and ParametersRule by always setting multiple to false.

Related Issue(s)

Closes #499

Please confirm the following checks

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream module

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 427/427}$ | ${\color{red}Fail: 0/427}$ |

[saibatizoku]

LGTM.

[apskhem]

LGTM