Review pooltestr-related docs

[fredjaya]

Updated the dedicated explanation of hierarchical sampling structure, and moved relevant text from the estimating prevalence tutorial to this page.

• This definitely should be reviewed and edited/improved
• Can contain as much technical information necessary

Added guide for how-to analyse pooled data. This is intended as a follow-along for users' own data.

• Could benefit with more information for why and when users' should use the different models
• Explanation of concepts (e.g. hierarchical sampling structure) should aim to be limited here

[fredjaya]

New page for How‐to prepare your data for analysis.

• Mainly textual edits required
• Check if language is accessible and not too technical

[fredjaya]

Edits made a while ago, thanks Angus!