Feature add hreflang x-default tag to sitemap

[RomainGueffier]

What does it do?

Add an additional link tag with hreflang x-default to improve locale detection fallback.
Add in settings form a select to save user preferences with 3 options:

• Disabled
• Default language pathname
• Other
  When last option is selected, an extra text input field shows up to put a custom url.
  Add translations for English and French
  Extend default Test collection to add slug field in order to have better use cases to try feature.

Why is it needed?

Improve SEO.

How to test it?

Run strapi with plugin installed. Open Sitemap plugin settings.

Outputs:

1. Disabled

2. Default Locale

3. Other

Related issue(s)/PR(s)

Closes #161

Tasks:

• add more translations ?
• can't find how to update docs via yarn docs:generate mentioned in CONTRIBUTING.md.
• improve labels and descriptions to help users understand what it does and SEO risks.

⚠️ yarn develop / yarn playground:develop did not update admin UI on save, whereas server was ok. Is it normal ? I needed to restart playground on each UI changes.

[boazpoolman]

Thanks for this PR @RomainGueffier.
Functionally it works perfect!

I just have a couple remarks on your code.
If you could implement those changes I'll get this merged and released asap :)

[boazpoolman]

⚠️ yarn develop / yarn playground:develop did not update admin UI on save, whereas server was ok. Is it normal ? I needed to restart playground on each UI changes.

You can run yarn playground:develop --watch-admin to make sure the admin also updates on save.

can't find how to update docs via yarn docs:generate mentioned in CONTRIBUTING.md.

Sorry. Those yarn docs: commands are outdated and should be removed from the package.json

[boazpoolman]

LGTM!