EPMDEDP-17216: docs: Add upgrade guide for KubeRocketCI 3.13 to 3.14

[geekhubuser]

Description

Adds an operator-guide page for upgrading KubeRocketCI v3.13 to v3.14
(docs/operator-guide/upgrade/upgrade-krci-3.14.md) and registers it at the top of the
Upgrade sidebar. It mirrors the structure and style of upgrade-krci-3.13 (admonitions,
title=-annotated code blocks, add-ons/Helm tabs, verification checklist).

3.14 is a feature release: the guide documents the optional, off-by-default Envoy Gateway /
HTTPRoute exposure, cancelInProgress review pipelines, the new clusterName configuration
for pipeline URLs, Service Account token login, and the expanded portal capabilities, and
states explicitly that no forced resource migrations or breaking Custom Resource changes are
required for this upgrade.

Ref: EPMDEDP-17216

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement

How Has This Been Tested?

• Technical details validated against the official edp-install 3.14.0 RELEASES.md and the
  published epamedp/edp-install 3.14.0 Helm chart.
• Off-by-default Envoy claim verified with helm template: a default 3.14.0 render produces
  0 HTTPRoute objects (nginx Ingress only); HTTPRoute appears only with
  httproute.enabled: true / ingressController: envoy.
• "No new prerequisites" verified by diffing default renders of 3.13.5 vs 3.14.0 (identical
  cert-manager Certificate / ValidatingWebhookConfiguration footprint).
• Value paths (clusterName, pipelines.cancelInProgress, gatewayApi, ingressController)
  confirmed against the 3.14.0 chart values.
• Docusaurus build validated

Checklist

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Pull Request contain one commit. I squash my commits.

Screenshots (if appropriate)

N/A — documentation-only change.

Additional context

New file upgrade-krci-3.14.md + a sidebars.ts entry (newest-first). Filename follows the
current upgrade-krci-<version> convention (3.12/3.13), not the legacy upgrade-edp- prefix.
The 3.14 versioned-docs snapshot is out of scope here (separate release task).

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