Gigaset: fix N870 documentation link

Reviewed-by: https://github.com/sduthil

WAZO-2406 / WP-360: add Gigaset N870

Reviewed-by: https://github.com/apps/wazo-production-sf

WAZO-2383: fix Yealink v81 firmware links

Reviewed-by: https://github.com/apps/wazo-production-sf

reason: the handsets use this frequency band only.

reason: N870 does not restrict the provd version to >= 0.2 like the N720, so
we need to make it compatible with previous versions.

reason: if the extension is shorter than 4 characters, we want a 4 character code anyway.

reason: v2.17.2 supports fixed handset numbering.

Why: no longer available

WAZO-2313: fix Yealink v82 firmwares

Reviewed-by: https://github.com/apps/wazo-production-sf

Why: not available anymore