[WEBSITE] Use local links for CONTRIBUTING and DISCLOSURE_POLICY

Summary:
After D7437 the files are accessible from the website, so use them.

Depends on D7437.

Test Plan:

make doc
make serve

Navigate to the contributing page (menu) and the disclosure policy
(footer).

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7438