Add documentation for CI variable GLCI_MEDIUM_RUNNER_REQUIRED

Description

This is a follow-up issue to create docs for the GLCI_MEDIUM_RUNNER_REQUIRED CI ENV var introduced in Add CI variable GLCI_MEDIUM_RUNNER_REQUIRED (!207000 - merged)

Here's the suggestion for adding the docs from this thread comment:

@cwoolley-gitlab Looking at the existing documentation structure, doc/development/pipelines/internals.md might be the most appropriate spot since it already documents the GLCI_ prefix and other GitLab project-specific CI variables.

It could fit after the Variable naming section - something like:

### Required CI variables

Some CI/CD jobs require specific variables to be defined to run. Unlike optional variables, if these required variables are not defined, the affected jobs are skipped entirely.

#### `GLCI_MEDIUM_RUNNER_REQUIRED`

Enables system tests that require runners with at least 4 cores and 16 GB RAM. Chrome version 133+ requires additional compute resources to run reliably, otherwise system test jobs become unpredictably unstable due to insufficient resources for the PostgreSQL database and Rails application.

When not defined (default is empty string), system tests requiring medium runners are skipped without failing the pipeline. This prevents resource-intensive tests from running in environments without sufficient runner capacity, such as community contributor forks.

Required for environments where system tests need to run:

- The canonical `gitlab-org/gitlab` project
- Security forks where system tests are needed  
- Development environments running the full test suite

The value should be a runner tag that points to runners with sufficient resources.
For example, `saas-linux-medium-amd64` for medium SaaS runners on GitLab.com.

We could add other required CI variables to this section as needed. WDYT?