[pages/languagedev_participation.html] minor updates on links/URLs

[himorin]

• I suppose we apply the same CoC (CEPC) and license over all groups, and in some groups these are only referenced from CONTRIBUTING.md. So having links here would help.
• Get a GitHub id link has two parameters (like pointing w3c/alreq as where user jumped from), but we need non of these.

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