Add docs around adding offline support for SDKS

[AbhiPrasad]

On a high level getsentry/sentry-dotnet#280, captures most of it.

• https://github.com/getsentry/sentry-electron/pull/392
• https://github.com/getsentry/sentry-dotnet/issues/1067https://github.com/getsentry/sentry-cocoa/issues/316#issuecomment-983737518
• https://docs.sentry.io/platforms/java/guides/spring/configuration/
• https://docs.sentry.io/platforms/javascript/configuration/integrations/plugin/#offline

Some more comments from Bruno:

• Make sure DSN is part of the path (a hash of it is fine), or alternative solution to isolating different apps using Sentry from sending the dump to a different project
• Try to send old/previous runs when starts up.
• Keep a max number of files on disk not to run out of space. Delete older dumps first (with session support we also move the init flag from one envelope to the next session update, hacky but it's the solution we have so far)

[bruno-garcia]

Additionally: An option to slow down between captures when reading off from disk (avoid triggering abuse filters in Sentry)