docs: Start markdown headers with capital letter where applicable

[Kotbenek]

This patch consistently capitalizes the first word of each markdown header where applicable.

[bagder]

I'm not convinced with need to insist on capitalization in headers. There are numerous additional markdown headers that also start with lowercase.

[Kotbenek]

Thank you for your insight.

There are indeed some markdown headers starting with lowercase, although they start with words like curl, and they are meant to be written with lowercase, I think.

Other headers, except the ones changed in the patch, are in fact fully capitalized (most of the docs), or fully in lowercase (BUFQ.md).

The thing that made me create this PR was seeing docs/INSTALL.md, beginning with lowercase how to install curl and libcurl, followed by capitalized headers - it just does not seem consistent to me.

[bagder]

Thanks!