Update PKCE-related documentation

[Widcket]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

Recently there have been some changes in Manage, and now the Token Endpoint Authentication Method setting no longer shows up for mobile apps (because for this kind of app it must be None). Whereas for regular web apps and SPAs it's been moved to another tab:

Since it was possible (before this change) to set it to an invalid value for mobile apps, we had to point this out in the README of the app-facing package. Now that this setting is no longer available for mobile apps and was moved for SPAs, this PR reworks the respective documentation.

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +2.51% 🎉

Comparison is base (d809aa0) 93.57% compared to head (a294d9a) 96.08%.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #300      +/-   ##
============================================
+ Coverage     93.57%   96.08%   +2.51%     
  Complexity       86       86              
============================================
  Files            70       97      +27     
  Lines           965     1611     +646     
  Branches         50      331     +281     
============================================
+ Hits            903     1548     +645     
  Misses           49       49              
- Partials         13       14       +1     

Flag	Coverage Δ
auth0_flutter	100.00% <ø> (ø)
auth0_flutter_android	96.55% <ø> (ø)
auth0_flutter_ios	99.84% <ø> (?)
auth0_flutter_platform_interface	87.03% <ø> (ø)

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see 27 files with indirect coverage changes

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