Cakephp 5.0.0-beta1 instructions

[Coreusa]

Add details for installing the cakephp 5-beta 1 in the installation docs.

[LordSimal]

CI is broken here because the merge conflict in en/appendices/migration-guides.rst has not yet been cleaned up.
But if #7565 is merged this PR can simply be rebased and it should be fine.

[Coreusa]

@LordSimal Yeah, seems this is related to files I haven't touched.

Excerpt:

/home/runner/work/docs/docs/en/appendices/migration-guides.rst:14: WARNING: Definition list ends without a blank line; unexpected unindent.

/home/runner/work/docs/docs/en/appendices/migration-guides.rst:14: CRITICAL: Missing matching underline for section title overline.

=======
    ./4-0-upgrade-guide
    ./4-0-migration-guide
/home/runner/work/docs/docs/en/appendices/migration-guides.rst:22: WARNING: Block quote ends without a blank line; unexpected unindent.

[markstory]

Thank you 🎉