feat: make useSession isomorph :confetti:

[BracketJohn]

Closes #21.
Contributes to nuxt/framework#30 (security section added)

This PR introduces isomorphic session fetching. Getting this to work was quite tricky as regular useFetch cannot / should not be used in composables and methods, prompting vue warnings, see nuxt/nuxt#12774

The solution was to:

• switch to $fetch
• write some utilities that produce correct urls, redirects, ... on the server- and the client-side

This PR also:

• makes the 2-provider credential example more realistic
• adds a basic security section
• adds a custom sign in page docs section

Checklist:

• issue number linked above after pound (#)
  • replace "Closes " with "Contributes to" or other if this PR does not close the issue
• manually checked my feature / checking not applicable
• wrote tests / testing not applicable
• attached screenshots / screenshot not applicable