Add explanation docs about APIs

[philippjfr]

In this PR I plan to add two different explanation sections:

• Reactivity vs Callbacks (Preview)
• Functions vs Classes

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 71.56%. Comparing base (ee579e6) to head (b5270ef).
Report is 3 commits behind head on main.

Files	Patch %	Lines
panel/param.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6289       +/-   ##
===========================================
- Coverage   81.91%   71.56%   -10.36%     
===========================================
  Files         310      310               
  Lines       45992    45996        +4     
===========================================
- Hits        37676    32917     -4759     
- Misses       8316    13079     +4763     

Flag	Coverage Δ
ui-tests	?
unitexamples-tests	71.56% <50.00%> (+0.08%)	⬆️

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[jbednar]

Looks good. For expedience I pushed some direct edits to the PR, so please review that commit and make sure I didn't break anything. :-) I think this guide helps, but I still think we'll need more info, e.g. "what are all the cases where I can provide a parameter object and it will be treated as a reactive object"? Clearly some Panel objects accept them for Parameter values; is it all Panel objects, for all Parameters?

[jbednar]

I'm not sure "discussion" adds much here.

[Coderambling]

Might misunderstand the above context, but I think I ran into the above issue when using layouts. To my surprise it was quite easy (for Philip at least :-) ) to update this in the Panel code for Flexbox. See below.

A number of ListLike layouts do not support objects = at the moment. The PR below would fix that and make behaviour more uniform.

I Made a PR for Flexbox, Philip, was able to fix it with +- 5 lines of changed code. At his request I made this PR for the other layouts: #6389