Software Heritage resolver info in first footnote

The most basic way to resolve a Software Heritage identifier (SWHID) is to
prefix it with 'https://archive.softwareheritage.org'. However, Roberto Di
Cosmo informed me that SWHIDs are also resolved by 'n2t.net' and
'identifiers.org'.

With this commit, on the first occurance of an SWHID, I added some
explanation of how to resolve it by adding 'http://n2t.org' (since it was
the shorter option).

Some further minor edits were made:

 - In the manuscript submission information, instead of "published on
   IEEE", I wrote "first published online". The journal name is available
   on the top of every page and doesn't include "IEEE", so this hopefully
   avoids some confusion for people who don't know CiSE is published by
   IEEE.

 - The URL with the link to Ubuntu images was moved to footnotes to help
   the readablity and better type-setting of the paragraph. A minor edit
   was then made in that paragraph to shrink the paragraph by two words
   that had occupied a whole line in its end.

 - The first comment line in the second listing (Git commands to start a
   new branch from Maneage) was slightly edited to avoid the term 'main'
   (which could be confused with the branch name after 'git checkout -b
   main').

 - In the acknowledgements, the paragraph on Maneage commit/branch
   information was moved at the top so the people and institutions are
   acknowledged immediately after each other.

 - Some minor edits were made in the Spanish acknowledgements to fit with
   new project names.

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