Added explanation for Ceil

[SpiderMath]

Description

This pull request aims to add an algorithm explanation for Ceil. I'm aware that this is a small addition but I plan to add further, but I have been told in the past to add things one by one, hence I'm following those instructions. I've tried to be as concise yet understandable as possible, but I may have overcomplicated things a bit, please do check and let me know what sort of changes you may require me to make in this added documentation.

Checklist

• A description of the changes in this PR is mentioned above.
• All the new and existing checks pass.
• No plagiarized, duplicated, or repetitive documentation that has been directly copied from another source.
• If it's a new explanation, it contains solid, understandable, and accessible information.
• I have read the whole contributing guidelines and agree to the Code of Conduct.

Screenshots (if any)

Not applicable in this case.

Note to reviewers

Already stated in description!