[Doc] Improve AuthProvider documentation

[slax57]

Problem

Even after reading the authProvider doc, it’s not easy figuring out how the redirection mechanism (with /auth-callback) works.

A diagram explaining the steps would be a huge help.

Besides, the authProvider doc was not reworked like the dataProvider doc was. It needs more love.

Solution

• Move sections dedicated to each method up by one level to make them easier to discover
• add a table at the beginning of each section to summarize whether or not it is required, for what purpose, what the parameters and the expected response and error are
• remove the sections previously containing these information but too far from the methods they apply to
• better explain when to use login vs handleCallback and canAccess vs getPermissions
• improve findability of the 'Using External Authentication Providers' section
• add a sequence diagram to better explain the 3rd party auth flow

How To Test

make docker-doc
make doc-headless

Todo

• Validate changes with OP
• Also apply changes to ra-core doc

Additional Checks

• The PR targets master for a bugfix or a documentation fix, or next for a feature
• The documentation is up to date

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