Refactor GB pages to use a single source of data

We have the same data about the governing board (GB) reproduced in multiple parts of the website, including committee chairs on https://github.com/matrix-org/matrix.org/blob/main/content/foundation/governing-board/index.md and https://github.com/matrix-org/matrix.org/blob/main/content/foundation/governing-board/committees.md. This can lead to the issue of desyncing as was the case in #2997.

We should refactor this to use a single source of truth. One way to do so could be how #2986 is implemented.

Start by comparing the governing-board data in content/foundation/governing-board/index.md and content/foundation/governing-board/committees.md, then read the approach used in #2986. Trace every duplicated GB value and verify that both pages use one shared source, with no remaining opportunity for the pages to desync.

Written by the indexing model from the issue text.