Style Guide: Information on cross-references

[FionaDyalog]

Amend the section on cross-references in the Style Guide to cover the following:

• Referring to a section on the same page
• Refererring to a page within the same project (and a section thereof)
• Referring to a PDF
• Referring to documentation in a different GitHub project

Information should cover:

• when to end the link with a / and when .md is needed
• how to create relative links (for example, only go back to docs.dyalog.com// and never include the version number itself within the link)

Pending decsion: whether we continue with PDFs beyond v20.0, as this will impact cross-document reference format.