Introduce the `lmdb_version` function

[postmeback]

Pull Request

Related issue

Fixes #199

What does this PR do?

• Adds a function in lib.rs, to expose the LMDB version.

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [x] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

Thank you so much for contributing to Meilisearch!

[postmeback]

Committed with your suggestion.

[Kerollmops]

Thank you!

Would it be possible to make the CI pass, add documentation and a documentation example, please?

[postmeback]

Can you give me an example, of where to write the documentation and example? @Kerollmops