Simplify and update the getting started documentation

[brooke-hamilton]

Benefit/Result/Outcome

Simplified getting started documentation, visible on a single page, optimized for people who are new to Mission Landing Zone.

Description

Update our deployment guidance to have a single Getting Started page that includes deployment instructions.

Remove references to the deployment container - it is less important than it used to be. (The container was highly useful for previous MLZ Terraform versions that set up a service principal for deployment. We no longer provide that feature.)

See #431 for additional context.

Acceptance Criteria

• De-emphasize the development container.
• Consolidate other deployment docs into the Getting Started where it makes sense. Decide whether to put all deployment instructions into the Getting Started document, or have a single menu-->instructions page links.
• Do not have three layers of links to find deployment instructions.
• Make sure the prerequisites are clearly defined up front.
• Ensure that the different deployment methods are clearly defined.
• Ensure that the optional deployment features are clearly documented, e.g. Sentinel and policy
• Consider consolidating Bicep and Terraform deployment instructions into the same docs, like MSFT docs pages which show tabs for different languages, like BASH vs. Powershell