ci: add issue templates for website bugs, enhancements and good first issues

.github/ISSUE_TEMPLATE/enhancement.md

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+---
+name: Enhancement
+about: Suggest an improvement to the site, such as performance, SEO, accessibility, or tooling.
+labels: kind/feature
+---
+
+**What you would like to be added**:
+
+**Why this would help**:
+
+**Anything already in place that this builds on**:

.github/ISSUE_TEMPLATE/good-first.md

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+---
+name: Good first issue
+about: Publish a task that is a good starting point for a first-time contributor.
+labels: good first issue
+---
+
+**Task description**:
+
+**Suggested solution**:
+
+**Files likely involved**:
+
+**How to verify**:
+
+**Who can take this**:
+
+Good first issues are reserved for first-time contributors. After one or two of them, move on to `help wanted` items and leave the remaining good first issues for the next newcomer.
+
+**How to take it**:
+
+Reply on this issue with `/assign` on its own line.
+
+**How to ask for help**:
+
+Ask here on the issue. The author or another community member will guide you through the contribution process.

.github/ISSUE_TEMPLATE/site-bug.md

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+---
+name: Website bug
+about: Report something broken on the site itself, such as rendering, search, links, or the build.
+labels: kind/bug
+---
+
+**Page URL**:
+
+**What happened**:
+
+**What you expected**:
+
+**Browser and locale** (if it is a rendering issue):