Add join function documentation to chart template guide

[Copilot]

Adds documentation for the join function in the chart template guide's functions and pipelines section.

Changes

• Added new "Using the join function" section after the default function section
• Includes function signature, practical example with list joining, and sample output
• Notes relationship to split function (inverse operation)

Example

# values.yaml
favorite:
  drinks:
    - coffee
    - tea
    - water

# template usage
drinks: {{ .Values.favorite.drinks | join ", " | quote }}

# output
drinks: "coffee, tea, water"

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add a mention of join in docs/chart_template_guide/functions_and_pipelines.mdx</issue_title> <issue_description>Add a mention and example of go text/template join in docs/chart_template_guide/functions_and_pipelines.mdx

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes helm/helm-www#1900
• Fixes https://github.com/helm/helm-www/issues/1334

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