Add a secondary (right) sidebar to the FastListTemplate

[dalthviz]

Hi, this PR attempts to add a secondary sidebar (on the right) to the FastListTemplate. This new sidebar works in a similar manner than the existing sidebar including variables/params to handle:

• Sidebar elements list: right_sidebar
• Sidebar width: right_sidebar_width
• Sidebar initial collapsed state: collapsed_right_sidebar
• Sidebar footer: right_sidebar_footer

Also added a simple test to check the new sidebar and updated the notebook related with the FastListTemplate to include the new variables/params.

Let me know if something else needs to be done!

A preview:

import panel as pn

pn.extension(sizing_mode="stretch_width")

CSS = """
div.card-margin:nth-child(1) {
    max-height: 300px;
}
div.card-margin:nth-child(2) {
    max-height: 400px;
}
"""

title = pn.pane.Markdown("# **Title**")
layout0 = pn.Column(
    pn.pane.Markdown("Content", styles={"color": "white"}),
    pn.VSpacer(),
    styles={"background": "blue"}, sizing_mode="stretch_both"
)

layout1 = pn.Column(styles={"background": "green"}, sizing_mode="stretch_both")
layout2 = pn.Column(styles={"background": "red"}, sizing_mode="stretch_both")
secondary_title = pn.pane.Markdown("# **Secondary title**")
layout3 = pn.Column(
    pn.pane.Markdown("Secondary Content"),
    pn.VSpacer(),
    styles={"background": "yellow"}, sizing_mode="stretch_both"
)

pn.template.FastListTemplate(
    site="Panel",
    title="App",
    sidebar=[title, layout0], 
    right_sidebar=[secondary_title, layout3],
    main=[layout1, layout2],
    raw_css=[CSS],
    sidebar_footer="<b>Footer</b>",
    right_sidebar_footer="<b>Secondary Footer</b>",
).servable()

Fixes #7696