antenna: Reformat sphinx doc and make it uniform to other docs

This MR reformats the Sphinx documentation of the Antenna module to comply with the #1139.

For comparison, take a look how it currently looks before the change: https://www.nsnam.org/docs/models/html/antenna.html

This is how it will look after the change:

• Section style follow the Sphinx style defined for ns-3.
• No messed-up menu on the left
• Concise
• References in the document have been reformatted to use numbers.
• The References point to each individual reference at the bottom.
• The References at the bottom in some cases include the link to the document they reference.

Several sections are still lacking content.

If @biljkus or @Gabrielcarvfer, who are more familiar with this module, could provide more information, it could improve the documentation. In particular Scope and limitations and Usage are missing content. Attributes and traces are not necessary as they will be automatically provided by another MR.

If providing more information is not possible now. Then it is ok to push "as it is".