docs: add contributing guide

[cgb102000]

Resumo

Adicionei um guia de contribuição ao projeto em formato profissional e amigável para iniciantes.

Alterações

• Criei o arquivo CONTRIBUTING.md
• Incluí instruções para fork, clone, branch, commit, push e PR
• Adicionei convenções de commits e checklist para contribuições

Motivação

Tornar o processo de contribuição mais claro e acessível para novos participantes.

[Navashub]

Hey @cgb102000 , thanks for the contribution - the pagination implementation in crud.py and the router updates are correct and match Issue #5 exactly. A few things to fix before this can be merged:

1. CONTRIBUTING.md has duplicated/conflicting content.
   It looks like merging main into this branch left the file with duplicate headers and sections that weren't cleaned up (two # Contributing titles, two intros, the Table of Contents appearing mid-document instead of at the top, etc.). Could you tidy this up into a single, clean version? Feel free to combine the best of both versions, but it needs to read as one coherent document.
2. This PR bundles two issues.
   The commits show this PR resolves both Issue #4 (CONTRIBUTING.md) and Issue #5 (pagination), but the PR description only mentions the contributing guide. Going forward, please keep one PR per issue, and update the description to reflect everything the PR actually changes -that's important for reviewers and for keeping the commit history readable for other contributors.
3. Branch naming.
   The branch is docs/contributing-guide, but it includes a feat: commit for pagination. Per our CONTRIBUTING.md guidelines, the branch name should match the issue being solved — for future PRs, please split unrelated work into separate branches/PRs.
   Once the CONTRIBUTING.md duplication is fixed, I'm happy to merge. Nice work on the pagination logic - that part is solid.

[cgb102000]

Updates the contributing guide to remove duplicated content and make the document clearer and more cohesive. It also reinforces the guidelines around branch naming, PR scope, and the checklist to follow before submitting a contribution.