Make publication badges always visible (alshedivat#2565)

## The issue
Currently Altmetric and Dimension publication badge elements have
non-obvious attributes that hide badges when some conditions are not met
,e.g.:
```
data-hide-no-mentions="true"
data-hide-less-than="15"
```
resulting in seemingly strange behavior where badges are enabled in
`config.yml` but don't show up consistently, as reported in alshedivat#2443 :
Altmetric badges don't display for some pubs.

## This PR
- removes these hidden nondisplay conditions in favor of more
predictable website behavior;
- adds documentation links to point users interested in customizing
badge behavior to the right resources.

_config.yml

@@ -348,9 +348,10 @@ scholar:
   group_order: descending
 
 # Display different badges withs stats for your publications
+# Customize badge behavior in _layouts/bib.liquid
 enable_publication_badges:
-  altmetric: true # Altmetric badge (https://www.altmetric.com/products/altmetric-badges/)
-  dimensions: true # Dimensions badge (https://badge.dimensions.ai/)
+  altmetric: true # Altmetric badge (Customization options: https://badge-docs.altmetric.com/index.html)
+  dimensions: true # Dimensions badge (Customization options: https://badge.dimensions.ai/)
   google_scholar: true # Google Scholar badge (https://scholar.google.com/intl/en/scholar/citations.html)
 
 # Filter out certain bibtex entry keywords used internally from the bib output

_layouts/bib.liquid

@@ -268,8 +268,6 @@
           {% if site.enable_publication_badges.altmetric and entry_has_altmetric_badge %}
             <span
               class="altmetric-embed"
-              data-hide-no-mentions="true"
-              data-hide-less-than="15"
               data-badge-type="2"
               data-badge-popover="right"
               {% if entry.altmetric != blank and entry.altmetric != 'true' %}
@@ -297,7 +295,6 @@
               {% else %}
                 data-pmid="{{ entry.pmid }}"
               {% endif %}
-              data-hide-zero-citations="true"
               data-style="small_rectangle"
               data-legend="hover-right"
               style="margin-bottom: 3px;"