Add topic guide describing rsun idiosyncracies

[alasdairwilson]

This takes some of the content from coords regarding rsun and its involvement in transforms/reprojects expands on it and makes a topic guide.

If anyone else can think of additional content that should be here and feedback on current content extremely welcome.

[Cadair]

Thanks! This is an excellent use of the topic guide.

[wtbarnes]

This will conflict with #6954. I would suggest merging that first since it does additional reorganization as well. It may be that all of the info in that (very incomplete) topic guide is completely redundant compared to the more complete information here.

[nabobalis]

The other PR is now merged.

[nabobalis]

Another really minor review

[nabobalis]

It unfortunately conflicts with main as well now.

[ayshih]

I don't want to block this PR, but here is the structure and content this topic guide eventually needs to have, which could be deferred to a follow-up PR:

• The title should be something like "Different values for the radius of the Sun"
• There ought to be an introduction that explains the radius of the Sun:

Observing in visible wavelengths
The meaning and context of the IAU constant
Observing in wavelengths other than visible (e.g., EUV)

• The second section should describe the rsun frame attribute

2D coordinates
Converting 2D coordinates to 3D coordinates, with examples

• The third section should describe how the radius of the Sun is determined for data files

RSUN_REF keyword, or calculated from RSUN_OBS keyword, or default
Why Maps from different data sources might have different `rsun` values for the same emission
Why the precise value of the radius of the Sun typically doesn't matter

• The fourth section then goes into reprojection

[nabobalis]

We should create a tracking issue for Alberys comment.