[FW][ADD] extensions: icon_role font awesome

[fw-bot]

Task: #3843168

This PR adds a custom extension for an :icon: role to allow the use of Font Awesome icons in documentation.

With the new role, icons can be called by their classname.

This is a Font Awesome :icon:`fa-step-forward` icon.

---

If this PR is approved, it will likely need to be manually forward ported to each version.

Forward-Port-Of: #8469

[robodoo]

Pull request status dashboard.

[fw-bot]

@samueljlieber cherrypicking of pull request #8469 failed.

stdout:

Auto-merging extensions/odoo_theme/static/style.scss
CONFLICT (content): Merge conflict in extensions/odoo_theme/static/style.scss

stderr:

17:06:48.551060 git.c:463               trace: built-in: git cherry-pick 9428a84da2148315a811254fcc89527cbe9edf33
error: could not apply 9428a84da... [ADD] extensions: icon_role font awesome
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[samueljlieber]

f9063ab resolved conflict on extensions/odoo_theme/static/style.scss.
@robodoo r+