Separate section for desktop user creation docs

[probakowski]

This change moves docs around automatic user creations in desktop access to its own section and also adds section about stored credentials.

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[public-teleport-github-review-bot[bot]]

@probakowski See the table below for backport results.

Branch	Result
branch/v13	Failed
branch/v14	Failed
branch/v15	Failed

[ptgott]

@probakowski Were you able to backport this manually? Thanks!