[Inkscape-translator] Website: New Extensions Page

Dear website translators,

we've got a new page on our website, written by Martin Owens, which gives some basic information about how to write an extension:

https://inkscape.org/en/develop/extensions/

Because of the new page, there are also a few other changes that need to be made:

- our community to-do list required an update (I've already done this for all languages that have one - relax ;) )

- the Develop page needs a small update https://inkscape.org/en/develop/ (see attachment, link deletions are not in the diff)

Thanks everyone for your work!

Kind regards, Maren