[Documentation]: Move algorithm descriptions from CLI manual pages to separate page

[edwardchalstrey1]

Requested documentation improvement

• Create a new RST page which uses the algorithms table as a starting point
• Link to this from the homepage clearly (a 5th purple link)
• Add a description for each of the methods, adapted from the current descriptions on the CLI pages but not CLI-specific
• Update table to link to all 3 of "Description", CLI & PyGambit func, where "Description" just takes you to a header further down the page
• CLI manual info should now just be how to run the method, and there is a link to a new page for the description
• Also add links to description from PyGambit pages

Consider opening as subsequent issues later

• Make each method a separately citable artefact
• Linked to [Documentation]: Tutorials demonstrating each of Gambit's equilibrium computation algorithms including those relying on external programs #561