Add a separate function to generate the PyGMT logo?

[seisman]

We're very close to finalizing the PyGMT logo design in #1404. #1404 (comment) shows that it's possible to produce PyGMT logos with pure PyGMT codes.

After finishing #1404, we need to decide what to do with the logo-plotting codes. We have some options

1. Create a standalone repository for PyGMT logos and store the codes in that repository
2. Add the logo-plotting codes as a gallery example
3. Add a separate plotting method Figure.pygmtlogo to produce the logo
4. Extend the existing Figure.logo method to produce the PyGMT logo

[yvonnefroehlich]

I think it would be cool to have a method pygmt.Figure.pygmtlogo within the pygmt.Figure() class to handle plotting the PyGMT logo into a Figure instance, similar to pygmt.Figure.logo for the GMT logo (or include this into pygmt.Figure.logo). And then have a gallery example like this:

fig = pygmt.Figure()
fig.basemap(region=[-5, 5, -5, 5], projection="X10c", frame=True)
fig.pygmtlogo(position="jMC", etc.)
fig.show()

The Python script in #3489 can be easly converted to a function

def pygmtlogo(
    color_concept="color",  # "color" | "bw"
    bg_concept="dark",  # "light" | "dark"
    shape="circle",  # "circle" | "hexagon"
    angle_rot=30,  # degrees
    wordmark=True,  # True | False
    orientation="horizontal",  # "horizontal" | "vertical"
    dpi_png=720,  # resolution of saved PNG image
):
...  # codes in 3489

pygmtlogo()
pygmtlogo(wordmark=True, orientation="vertical")
pygmtlogo(bg_concept="light", wordmark=True, orientation="vertical")

However, I would definitly need help to modifiy and convert these codes to a method within the pygmt.Figure() class.