Add documentation for stage providers

[lirenjie95]

Fixes #491

Add documentation for the 8 stage providers that were missing docs, under docs/symphony-book/providers/stage-providers/:

• mock — echo inputs to outputs, foo counter
• http — outbound HTTP calls with successCodes/wait.* polling
• create — create/remove solution versions and instances
• list — list instances/sites/catalog versions
• materialize — create objects from catalog versions
• patch — patch solution versions from a catalog version or inline content
• script — run bash/PowerShell scripts (local or remote)
• wait — wait for objects to be created

Each page follows the structure of the existing delay.md; Inputs/Outputs were verified against the Process implementation and tests of each provider.

Note: the remote provider is not included in this PR — happy to add it in a follow-up if desired.

[Haishi2016]

These are great but we are trying to move docs to Hugo doc under the docs-site folder.