Cation documentation

[Pepe-Marquez]

This is adds additional documentation

Summary by Sourcery

Enhance the documentation for perovskite compositions and ions in NOMAD by adding comprehensive guides and explanations of data schemas, improving user understanding and usability.

Documentation:

• Expand documentation on creating and managing perovskite compositions and ions in NOMAD, including detailed instructions for various tasks such as creating, uploading, and downloading compositions and ions.
• Introduce new documentation sections explaining the data schemas for perovskite compositions, ions, additives, and impurities, providing detailed descriptions of data fields.

[sourcery-ai[bot]]

Reviewer's Guide by Sourcery

This PR significantly enhances the documentation structure and content for perovskite-related functionality in NOMAD. It reorganizes existing content and adds comprehensive documentation about perovskite compositions, ions, and data schemas. The changes include new markdown files with detailed explanations and embedded tutorial videos.

No diagrams generated as the changes look simple and do not need a visual representation.

File-Level Changes

Change	Details	Files
Expanded and restructured perovskite composition documentation	Added new sections for creating, downloading, and uploading perovskite compositions Included embedded tutorial videos for various tasks Reorganized content with clear task-based sections	docs/how_to/create_a_perovskite_composition.md
Enhanced ion database documentation	Added overview of perovskite ions database functionality Created sections for creating, downloading, and sharing ion data Included embedded tutorials for ion-related tasks	docs/how_to/add_a_new_ion.md docs/how_to/export_and_explore_ion_data_and_structure.md
Added detailed schema documentation	Created comprehensive documentation for perovskite composition schema Added documentation for additives and impurities data schema Added documentation for perovskite ions schema	docs/explanation/perovskite_composition_schema.md docs/explanation/additives_and_impurities.md docs/explanation/perovskite_ions_schema.md
Updated main documentation index	Reorganized how-to guides section Added new explanation section entries Updated navigation structure	docs/index.md

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