fix documentation links in "See also" sections

[striezel]

Description

This PR fixes broken links in the "See also" section of various documentation pages.

References

Fixes #591.

Compare:

• Current state: https://boostorg.github.io/gil/html/design/point.html
  • links in "See also" are broken (404)

vs.

• fixed state: https://striezel-stash.github.io/gil/html/design/point.html (or https://web.archive.org/web/20220503120037/https://striezel-stash.github.io/gil/html/design/point.html for a snapshot at the time of the PR's creation)
  • links in "See also" lead to reference pages

Tasklist

• [x] Ensure all CI builds pass
• [x] Review and approve

[mloskot]

Thank you both!