[DOCS] Modernize and streamline Composer migration instructions

[garvinhicking]

The current documentation on Composer migration is a bit outdated (aimed at v9, v10 more than v12) and at some places not newbie-friendly enough with too little information.

This patch is a first effort to streamline this. Feedback appreciated, feel free to directly commit+push suggestions to this PR.

Releases: main, 12.4

[garvinhicking]

(Note: I haven't rendered the docs yet, don't know if I introduced parsing errors - I don't have the "old rendering" properly set up here to easily verify/test it)

[github-actions[bot]]

The backport to 12.4 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

``` error: could not apply 6808b91... [DOCS] Add interlinks, unify Packagist name hint: After resolving the conflicts, mark them with hint: "git add/rm ", then run hint: "git cherry-pick --continue". hint: You can instead skip this commit with "git cherry-pick --skip". hint: To abort and get back to the state before "git cherry-pick", hint: run "git cherry-pick --abort". ```

stdout

``` [backport-355-to-12.4 7f69496] [DOCS] Modernize and streamline Composer migration instructions Author: Garvin Hicking Date: Tue Jan 16 14:29:41 2024 +0100 5 files changed, 261 insertions(+), 200 deletions(-) delete mode 100644 Documentation/MigrateToComposer/BestPractices.rst [backport-355-to-12.4 d960bd3] Update Documentation/MigrateToComposer/MigrationSteps.rst Author: Garvin Hicking Date: Thu Jan 18 12:08:40 2024 +0100 1 file changed, 1 insertion(+), 1 deletion(-) [backport-355-to-12.4 5be74d1] Apply suggestions from code review Author: Garvin Hicking Date: Thu Jan 18 12:15:08 2024 +0100 2 files changed, 25 insertions(+), 25 deletions(-) [backport-355-to-12.4 c85d629] [DOCS] Implement suggestions made in review Author: Garvin Hicking Date: Thu Jan 18 16:40:09 2024 +0100 4 files changed, 108 insertions(+), 89 deletions(-) Auto-merging Documentation/Settings.cfg CONFLICT (content): Merge conflict in Documentation/Settings.cfg ```

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-12.4 12.4
# Navigate to the new working tree
cd .worktrees/backport-12.4
# Create a new branch
git switch --create backport-355-to-12.4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick c5e21693e78301368fc740a7f3f8b0a51113d682,84b7830dea1c91b7e913f862f3c2656db93bb1ac,947b25c0783b0467ec72a26184aa25ffd958a6ab,2f737b6f3fd1407b7c10bfa1455cce1a93a2dbf1,6808b91e4608a1ad21bd49d50585a296310a7b87,bce03dc767727f18003e8cf0b1d39670fc0cf56a,4a5755f60961a553f8104a34bc9eaaf7176708ef,ffa1bfee249b0b1385d3f38e8360a250c4efd557,34dee40ae343d97dc3c90c12b85e9a6441cf6082,b1262d23e45acc7de1ea9b974e2df7f07046a1cd,008e9073553613bb1619f2b127f6d1f575fea3ad,2b3671e6940bac81634494fe714d0666f5d911bb,73a0bcce5ec11c1bf551e981b0126f8787642830,1cc172ca02e0edffac2d934e45b3bc88e9cd04ff,3f494527775120051cb643ef59cd2c4dcdf30835,d76a20c7ac254ab557b72283a5b790f29c05b6f9,09126d252ca25f3271cddbe7b30fe82ceb2ff03c,30b109b943dc48b05b8bddae09af457ab62c851d,b992b0e0d7a7b0602c172fec90e497d9da08dad3,3c71e2365ddda6003c48deeee3e8031ff21d1678,f7b44b116479bb950e735843a70ba1dc58afa656,c9fdf59e9024531a35f97cf1e370975aecba806d,c6eff202bcfece1500b4dc3d610021914a7d6dbd,b0e85aa2f56733c496d0848f79f05defb16bd348,ef930eb8762b380aa423746286ab7cb59e3320b5,3030d65ee8e0f186b361ce47e0662d3140336968,181a7879ea6305d14316c67ffa93fa936531f30c
# Push it to GitHub
git push --set-upstream origin backport-355-to-12.4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-12.4

Then, create a pull request where the base branch is 12.4 and the compare/head branch is backport-355-to-12.4.