Added shadow entries

[indicatelovelace]

I added a 'shadow' option for all entry functions. These are entries, that do not count as the first entry. This is useful e.g. when an abbreviation somehow lands in an outline, where you do not want the first entry to be.
I added an example, so take a look yourself.
I also modified the README.

[quachpas]

Hello! I'll be taking a look and thinking on this. Thanks!!

[quachpas]

I am unsure about this technical solution. I am of the opinion that any use of a glossary entry should raise the first use flag, and any subsequent issues with other elements can be solved without the user's manual intervention, that is setting and option to a XglsY call. Provided the main issue is

This is useful e.g. when an abbreviation somehow lands in an outline, where you do not want the first entry to be.

we can always define default show rules, so that glossary entries do not link and label inside outline.entry, figure.caption, etc. and more importantly make it seamless to the user, with a specific shadow suffix to keep the link, but not raise the first use flag.

Users can also manually use functions such as gls-short, gls-long. By default, they do not link.

The default show rules solution seems more elegant to me, what do you think?