Fix content bugs found in Terminus docs audit

[EdwardAngert]

Summary

Stacked on #1 (data refresh) and pantheon-systems#10177 (automation mechanism). Concrete, verified issues from a full-set audit of src/source/content/terminus, cross-checked against a clone of pantheon-systems/terminus where the finding depended on real command/config behavior.

• 07-create.md: invalid composer.json in two examples (backslash instead of forward slash in a package name; missing comma between extra and require-dev blocks), a namespace mismatch between the documented distribution namespace and the composer.json autoload examples, a mismatched code-fence length that would break rendering, a mislabeled .gitignore code block, and stale example values (PHP 7.0.11, Terminus ^1.1/^3 compatible-version constraints against a current major of 4.x).
• 09-configuration.md: clarifies that hide_git_mode_warning (config.yml) and TERMINUS_HIDE_GIT_MODE_WARNING (env var) are the same switch — confirmed via TerminusConfig's TERMINUS_<KEY> env-var mapping in the Terminus source, which the doc never explained.
• 05-scripting.md: clarifies TERMINUS_SITE is a real Terminus-recognized env var (confirmed in src/Hooks/SiteEnvLookup.php, fires on every command with a site/site_env argument), unlike the self-defined ORG_UUID/PANTHEON_ORG examples nearby.
• 11-updates.md: fixed a self-referential link (pointed at its own page instead of the Terminus 4.0 migration doc) and added links to both major-version migration docs, which were otherwise unreachable from the page readers actually land on.
• 12-terminus-3-0.md: added a forward-pointer to the 4.0 migration guide now that the doc is reachable again.
• 10-supported-terminus.md: fixed a heading hierarchy skip (body text straight to an H3 with no H2 in between).
• 06-plugins.md: added missing cross-references to the plugin directory and create-a-plugin docs.
• ci/circleci.md: fixed an instruction telling readers to find-and-replace a literal TOKEN string that doesn't exist in the script; the other three CI docs already describe this correctly as a CircleCI env var.
• ci/bitbucket.md, ci/github-actions.md, ci/gitlab.md: fixed a repeated "how to to authenticate" typo.
• 03-examples.md: fixed two internal anchor links pointing at headings that don't exist on the page.
• 01-introduction.md: removed an unnecessary "powerful tool" flourish.

Intentionally out of scope: the CircleCI 1.0 test-automation example in 07-create.md is stale as a whole (CircleCI 1.0 is long deprecated) and needs a fuller rewrite, not a targeted fix; the placeholder site-name inconsistency in 03-examples.md and its duplicated "Basic Format" block with 04-commands.md are consolidation calls better suited to a follow-up pass.

This PR targets the fork for the same stacking reason as #1; re-target or reopen against upstream once the branches below it land there.

Reviewer action needed: reviewed frontmatter

src/source/data/schema.yaml defines reviewed as "the date this file was last reviewed by a member of the docs team" — a human-provenance claim, not a generic last-touched timestamp. This PR fixes real content in 03-examples.md, ci/circleci.md, ci/bitbucket.md, and ci/gitlab.md (all of which already carry a reviewed date, now stale relative to these fixes), and in 05-scripting.md, 06-plugins.md, 07-create.md, 09-configuration.md, 10-supported-terminus.md, 11-updates.md, and 12-terminus-3-0.md (none of which have the field at all). I deliberately did not set or update reviewed on any of them — that's not mine to attest to. Whoever reviews this PR should update reviewed to today's date on the files they've actually reviewed as part of merging it.

Test plan

• Extracted every JSON code block from 07-create.md programmatically and validated with json.loads (not hand-checked) — all 3 valid
• Grepped all TerminusHello* occurrences to confirm namespace consistency across the file
• Confirmed heading anchors (#basic-format, #apply-updates, #login-via-machine-token-required) exist and match the site's actual rehype-slug slugging behavior
• Confirmed all internal permalinks referenced (/terminus/create, /terminus/directory, /terminus/terminus-4-0, /terminus/terminus-3-0) against target files' frontmatter
• Re-verified TERMINUS_SITE/hide_git_mode_warning claims directly against Terminus source (SiteEnvLookup.php, TerminusConfig.php, SSHBaseCommand.php)
• Repo-wide grep confirms zero remaining "how to to" typos
• Reviewer updates reviewed frontmatter on files they've reviewed (see above)

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