[FW][FIX] base: fix link to non-existing documentation page

[fw-bot]

The documentation page for the external API was moved elsewhere with PR
odoo/documentation#2026.

See also:

• [FIX] redirects: add missing redirect rule for external_api.rst documentation#2251

Forward-Port-Of: #94452
Forward-Port-Of: #94122

[robodoo]

[fw-bot]

Ping @AntoineVDV cherrypicking of pull request #94122 failed.

stderr:

20:24:55.840068 git.c:344               trace: built-in: git cherry-pick 629449645f562f8f339f31ad61a9bb394f81222f
error: could not apply 629449645f5... [FIX] base: fix link to non-existing documentation page
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'
----------
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.

[AntoineVDV]

@robodoo r+