docs: Update metadata/custom-fields screenshots. Fix broken links

[rolodato]

Thanks for submitting a PR! Please check the boxes below:

• I have added information to docs/ if required so people know about the feature!
• I have filled in the "Changes" section below?
• I have filled in the "How did you test this code" section below?
• I have used a Conventional Commit title for this Pull Request

Changes

• Remove unused docs screenshots
• Update screenshot that still referenced "Metadata" instead of "Custom fields"
• Fix broken internal link to custom fields when no custom fields are configured
• Fixed grammar in message when no custom fields are configured
• Tweaked wording in custom fields UI

How did you test this code?

Manually, browse docs and custom fields pages locally

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Name	Status	Preview	Comments	Updated (UTC)
docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 22, 2024 8:34pm
flagsmith-frontend-preview	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 22, 2024 8:34pm
flagsmith-frontend-staging	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Aug 22, 2024 8:34pm

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Docker builds report

Image	Build Status	Security report
ghcr.io/flagsmith/flagsmith-api-test:pr-4534	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-e2e:pr-4534	Finished ✅	Skipped
ghcr.io/flagsmith/flagsmith-api:pr-4534	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith:pr-4534	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-private-cloud:pr-4534	Finished ✅	Results ✅
ghcr.io/flagsmith/flagsmith-frontend:pr-4534	Finished ✅	Results ✅

[github-actions]

Uffizzi Preview deployment-55491 was deleted.