Update internal and external documentation for 4.1.x and 5.0.x (add Extending and Developing page, update project page content) [#3538837]

Problem/Motivation

The Environment Indicator documentation has not been updated to reflect the new features and structural changes introduced in the 4.1.x and 5.0.x branches. These include:

Modernized CSS architecture using CSS variables for colors, borders, and sizing.
New environment_indicator_toolbar submodule replacing the old built‑in toolbar integration.
Additional permission: view environment indicator current release for finer access control.
Optional $settings['environment_indicator_unrestricted_access'] flag for bypassing permission checks in certain contexts.

The current documentation does not mention these changes, which may confuse site builders and developers trying to use or integrate the module.

Proposed resolution

Update the module documentation on Drupal.org to reflect the new features and structure:

Project page: Add note about upgrading from older versions.
Internal: Update https://git.drupalcode.org/project/environment_indicator/-/blob/5.0.x/en... for any changes.
Types of Indicators and Integrations: Update to document the environment_indicator_toolbar submodule and note the removal of the legacy toolbar integration.
New "Extending and Developing" page:
- Document how to use the environment_indicator.indicator service to retrieve the active environment programmatically.
- List available CSS variables for theming and provide guidance for overriding them.
- Outline submodule structure to help developers add new integrations.