docs: Use double quotes for pip extras so install commands work in cmd.exe

[LHMQ878]

Closes #2232

Problem

The install commands in the READMEs single-quote the extras, e.g.
pip install -e 'packages/markitdown[all]'. cmd.exe does not strip single quotes, so pip receives
them as part of the path and rejects it:

ERROR: 'packages/markitdown[all]' is not a valid editable requirement. It should either be
a path to a local project or a VCS URL (beginning with bzr+http, ...).

Change

Switched the six pip install snippets in README.md and packages/markitdown/README.md
from single to double quotes.

Double quotes rather than dropping the quotes entirely, because the quoting is still needed on
POSIX shells - [all] is a character-class glob pattern in bash/zsh, so an unquoted
markitdown[all] can be word-expanded if a matching path happens to exist. Double quotes are
the one form that works in cmd.exe, PowerShell, bash and zsh alike.

Verification

Windows 11, Python 3.12, pip 26.1.2. Using --dry-run --no-deps so nothing is actually
installed:

command	cmd.exe	PowerShell
pip install -e 'packages/markitdown[all]' (before)	ERROR: not a valid editable requirement	ok
pip install -e "packages/markitdown[all]" (after)	ok	ok

Note that this only reproduces under cmd.exe. PowerShell strips single quotes itself, which
is probably why it has gone unnoticed.

After the change:

• No single-quoted extras remain in any markdown file in the repo.
• All six updated snippets resolve correctly; invalid editable requirement errors: 0.
• No CI workflow or shell script referenced the quoted form, so nothing else needed updating.

The reporter suggested removing the quotes; double-quoting keeps the POSIX side safe as well,
but happy to switch to unquoted if you prefer that for consistency with other docs.

[LHMQ878]

@microsoft-github-policy-service agree