Add examples to function documentations

[loreabad6]

Include examples for the sfnetworks functions.

[loreabad6]

Progress:

• [x] attrs.R
• [x] blend.R
• [x] edge.R
• [x] join.R
• [x] morphers.R
• [x] node.R
• [x] plot.R
• [x] sf.R
• [x] sfnetwork.R
• [x] shortest_paths.R
• [x] tibble.R

[Robinlovelace]

This is a great summary of what needs doing here.

[luukvdmeer]

Lots, lots and lots of examples added to many different functions. Thanks @loreabad6