Improved Home and Features page, add three new pages

[PhilReedData]

Resolves issues #8 , #9 , #10 , #11 , #12 , #13 , #14

• Update the Features page to mention gatekeepers, default policies/licences, API, programmatic access, Bioschemas, SPARQL endpoint (briefly). Link to all the feature subpages.
• Update the Home page to include a new row of the three new feature subpages, and reflect the updates of the features below.
• Update the Yellow/Who page to include ORCID and ROR.
• Update the Models page to include Morpheus and COPASI.
• Update the Explore and Annotate page to remove annotations and say more on exploring; rename file.
• New feature page for Extended Metadata.
• New feature page for Publishing and DOI
• New feature page for API and Programmatic Access

This PR does not include work on the Semantic Spreadsheets Rightfield page. Wait until further development with FAIR Data Station which will supersede this. #15

Selection of screenshots

[stuzart]

Just a minor comment, but otherwise looks good as an incremental improvement.

Idea for the future: each feature page could have a definitiion in the frontmatter that links to an appropriate page in the documentation, and links to it in a consistent way.

[stuzart]

could also mention powerpoint and CSV files