Fix typos, grammar and improve wording ("Installation" & "First project setup" chapters)

[glmvc]

I hope its fine to make this kind of pull requests :)
Just small things I noticed while reading the Getting Started documentation.

[linawolf]

Thanks a lot, this is very helpful! I just fixed the syntax and merging conflicts and have one little remark, then it is fit to merge. We should backport to 13.4

Automatic backporting to 12.4 will not be possible as you also changed the site management document which changed since them.

As a general tip: If you keep your changes small and limited to one topic, automatic backporting is easier.

[glmvc]

@linawolf Sorry, I wasn't aware of backporting... I will keep the commits for prs small next time :) Thanks for the fixes and the tip!

[github-actions]

The backport to 13.4 failed:

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

stderr

error: could not apply 05559a5... Update Index.rst ("Maintaining TYPO3")
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", 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".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging Documentation/Installation/Updates/Index.rst
CONFLICT (content): Merge conflict in Documentation/Installation/Updates/Index.rst

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-13.4 13.4
# Navigate to the new working tree
cd .worktrees/backport-13.4
# Create a new branch
git switch --create backport-683-to-13.4
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 05559a5fd1f8f93d97d4c74aee6c324700997606,2bd999f16c5545d0a7b02118c3fcbe15c1f4dc96,e20450cee9b6328ac7750ab64837af410d64279c,8c3bf62cabecce146874da5a368842d6eb27a4ca,afec1324197c02c49ad37139a990cdd81b61fbe0,94ff6335603181da7fca3f3c003184b50d54fb74,a98a7e98a3de87d9a4f9293fc98c6c567d359883,1800dc8525e1f001cafeaf54f23de033cecc0df0,47bb340097c5348998aafc5a06e49f3e8502b15a,2f053dfe871c035861bbf86601e171a619d83f3d
# Push it to GitHub
git push --set-upstream origin backport-683-to-13.4
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-13.4

Then, create a pull request where the base branch is 13.4 and the compare/head branch is backport-683-to-13.4.