Add persistence transform example to the gallery

[wafels]

The gallery should include some examples of simple operations on maps.

These include persistence transform maps and mapcubes and overplotting of multiple maps in the same matplotlib Axes instance.

Although these examples are relatively simple, they represent common operations in solar physics.

Here is a draft for the persistence transform - a gist

[prateekiiest]

Hacktoberfest has ended @nabobalis

[nabobalis]

I believe it comes around every year.

[prateekiiest]

ok, for next year :)

[florist-notes]

@nabobalis @wafels I would like to work on this issue.

[nabobalis]

@SKKSaikia You don't need permission to work on an issue.

Just be aware that there is a PR open (https://github.com/sunpy/sunpy/pull/2455) that deals with part of this issue.

[prateekiiest]

Gamma-curve transformation of a map - New Example

Can we work on that? Its pretty basic otherwise

[prateekiiest]

PR sunpy/sunpy#2455

Implemented

• [x] Base Difference
• [x] Running Difference
• [ ] Persistence Transform (Not to be ported here, but we can use it later) - The gist