[WEBSITE] Make the disclosure policy link absolute

Summary:
The current link is a relative link, so it only works from the website
top level.

Test Plan:

make doc
make serve

Navigate through the disclosure policy from the footer twice.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7464