Use bootstrap shadow/border classes for img

See merge request !152

Instead of setting box-shadow and border properties, use @extend to
inherit bootstrap pre-defined styles (shadow and border).

Improve the distinction between images and text

See merge request !149

Clarify differences between Matrix and XMPP

See merge request !150

First, replace Jabber with XMPP. The protocol was named Jabber in the
first place, but has been renamed to XMPP since then.

Also, make it clear that XMPP is also federated, and that federation is
not the reason why rooms are duplicated, this is just a particularity
of Matrix compared with XMPP.

In most documentation pages, the screenshots have a white background,
which makes them uneasy to distinguish from the text.

Add a CSS rule for all screenshots in order to add a margin on the
left, and a thin black border around. There is currently no custom CSS
file, so create one in "assets" directory.

For all figures with width="100%", reduce them to "90%", so that they
don't overflow on the right (and make a horizontal scrollbar appear),
and we have the same margin (5%) on the left and right sides.

With:
```shell
find . -path "./node_modules" -prune -type f -o -name "*.md" -o -name "*.html" | xargs perl -pi -e 's/ (?=«)/ /g'
```

With:
```bash
find . -path "./node_modules" -prune -type f -o -name "*.md" -o -name "*.html" | xargs perl -pi -e 's/( (?=[:!?;»])(?!!\[)(?!:=))|((?<=«) )/ /g'
```

Services - Add link to services

See merge request !145

Typo fix

See merge request !144

Doc add ntfy update procedure

See merge request !143

Update Nexcloud doc about Circles app

See merge request !141

With:
```bash
find . -path "./node_modules" -prune -type f -o -name "*.md" -o -name "*.html" | xargs perl -pi -e 's/ (?=[:!?;])(?!!\[)(?!:=)/ /g'
```

Replaced logo.svg → logo.png

See merge request !136

Added screenshot.png

See merge request !135

First draft of ntfy service page

See merge request !134

Fix broken link

See merge request !133