Add most recent release `v0.26.x` and `main` as the only options for
now as we've updated the structure and links to ensure they work
correctly on both GitHub and the website.

Add configuration for chain docs.
Front matter is missing so adding some basic one just to test the
PR. The docs include images at least one image (the logo) which
does not work as the only mime type supported for in-repo data is
text / markdown - which is something we need to fix in the sync
script.

This also adds support for using "." to get documents from the
repo root.
Signed-off-by: NAndrea Frittoli <andrea.frittoli@gmail.com>

displayOrder and docDirectory are not used anymore by the sync
script, so remove them from config, docs and tests.

The default target is now "" and not "/" anymore, so fix a few
comments where it referred to the old value.
Signed-off-by: NAndrea Frittoli <andrea.frittoli@gmail.com>

We used to check for reference names in the sync script, which required
the branch name to be configured as "origin/<branch-name>".
This approach broke some of the links.
Fix the sync script by looking for remote ref names in the python git
wrapper, and changing all config to use branch name only.

Fixes #313
Signed-off-by: NAndrea Frittoli <andrea.frittoli@gmail.com>

Signed-off-by: NShivam Mukhade <smukhade@redhat.com>