Fix broken links

After (proposed) inclusion to hifis.net!501, there are some broken links detected by jekyll's link checker in this repo. Fix that.

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• public/doc/helmholtz-aai/concepts/index.html:

  linking to internal hash #the-sp-idp-proxy-concept that does not exist (line 2421)
  * <a href="./#the-sp-idp-proxy-concept">Proxy Concept</a>

• public/doc/helmholtz-aai/howto-services/index.html

  mailto://ds-support@fz-juelich.de contains no email address (line 2460)
  <a href="mailto://ds-support@fz-juelich.de">contact us</a>

• public/doc/software/index.html

  image ../images/hzdr_logo.svg does not have an alt attribute (line 2455)
  image ../images/ufz_logo.png does not have an alt attribute (line 2456)

• public/doc/technical-platform/TP_Whitepaper/index.html

  linking to internal hash #biblio_node_package_manager that does not exist (line 2936)
  <a href="#biblio_node_package_manager">npm</a>

• public/doc/technical-platform/index.html

  linking to internal hash # that does not exist (line 2294)
    <a href="#" class="md-top md-icon" data-md-component="top" data-md-state="hidden">
        <svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8v12z"></path></svg>
        Back to top
    </a>