docs: fix broken links in section index pages

[SatvikMishra08]

Several links in the "In this section" lists point to slugs that don't exist, so they 404 on gin-gonic.com:

• binding: default-value → bind-default-values, collection-format → collection-format-for-arrays, custom-unmarshaler → bind-custom-unmarshaler, bind-html-checkboxes → bind-html-checkbox, custom-struct-tag → bind-form-data-custom-struct-tag
• middleware: using-basicauth → using-basicauth-middleware, goroutines-inside-middleware → goroutines-inside-a-middleware, ./custom-http-config/ → ../server-config/custom-http-config/ (that page lives under server-config)
• server-config: lets-encrypt → support-lets-encrypt, multiple-service → run-multiple-service
• server-config/websocket: /docs/en/docs/server-config/custom-http-config/ → /<lang>/docs/server-config/custom-http-config/
• blog: the Gin 1.11 announcement linked ../releases/release111.md, which resolves under /blog/news/; it now points to ../../releases/release111/

The same index pages exist in every locale, so I applied the fixes to all of them. I checked every new target on the live site.

This also covers the one-line fixes in #402 (BasicAuth) and #418 (custom unmarshaler).